Like the other answers, I also can not find an earlier usage of "holy Christian Church" before Luther. Therefore the rest of this answer tries to answer the sub-question
Was the Apostles Creed something that the Reformers tried to reform?
First, it seems that the word switch happens only within the Lutheran churches. A quick survey of other major Protestant denominations shows that they still use the word "catholic" (with a lower case 'c') but they recently added footnote to explain their meaning of "catholic" (see Reformed example and Methodist example).
Luther's understanding of 'catholic'
So let's examine whether Luther himself consciously provided a theological explanation for switching the word 'Catholic' to 'Christian'. I found a good 2013 journal article The Gospel Luther’s Linchpin for Catholicity by a Lutheran scholar Gordon Jensen.
The article explores again "how Luther defined and used the words catholic and Christian within the context of his understanding of ecclesiology and its apostolic task" in contrast with some extreme Lutherans's understanding of "holy Christian Church" in the creed today to mean "we are not catholic!"
The bulk of the article traces various letters, books, and documents by Luther as well as how other late medieval sources in Germany translated the Latin "catholic" as "Christian":
Luther’s claim that he is simply following custom is verified in some of the most popular works of practical divinity circulating in the empire on the eve of the Reformation⁹. This practice is followed in the popular Vocabularius predicantium, a handy dictionary that translated various biblical and ecclesiastical terms (mainly from Latin) into German, compiled by Johannes Melber and the Heidelberg humanist Jodocus Eichmann, and published frequently between 1480 and 1505.¹⁰ Under the entry “Catholic” the translation was given as “a Christian person.”¹¹ So Luther is not the first to make this switch.
Other late medieval sources in Germany also translated the Latin “catholic” as “Christian.” The most popular preaching manual in use in Germany on the eve of the Reformation was the Manuale curatorum of Johann Ulrich Surgant.¹² This Manuale offered both German and French translations of the Apostles’ Creed, since the book was designed for use in the area around the Rhine River. Surgant’s French translation of “I believe in the Holy catholic Church” (“credo in ecclesiam catholicam”) is “la saincte eglise catholique,” but his German translation reads “die heilige christenliche kirch.”¹³ So even before the Reformation, the word Christian was considered a more natural and appropriate to the German language than catholic, but this was not the case in French. The choice of word in these contexts was linguistic, not theological. Luther himself appears to support this practice of different translations for different languages. For example, until the end of his life, when he wrote in Latin, he continued to use the phrase, sanctam catholicam ecclesiam.¹⁴ He did not, therefore, reject the idea of the church catholic by his translation of catholic as Christian in the German language.
... But it is not as simple as this. He also took advantage of this golden opportunity to draw attention to the mistaken interpretation of this phrase by the pope, suggesting that his translation was also motivated by theological and political considerations.
To explore Luther’s theological understanding of catholicity, therefore, it is helpful to turn to his writings in the 1530s as he worked to shape and implement a reformation church. Three documents will be considered. ...
Far from being anti-catholic, as is all too often assumed, Luther was a strong
proponent of the church catholic, the holy Christian church—as long as one accepts
his understanding of the church catholic. It is not the unity of the church under Rome, nor the institutional structure that makes the church catholic. Rather, the assembly is catholic when the gospel is found in its midst. Catholicity is therefore connected to apostolicity. The gathering of people alone cannot make it catholic. Catholicity comes from the proclamation of the apostolic message, which transforms the gathering into the gathering of God’s people. The unity of the church is rooted in the one gospel, in which there is “one Lord, one faith, one baptism” (Eph 4:1–5).
In his book, Martin Luther: Prophet to the Church Catholic, James Atkinson observed that:
It is manifest from all the evidence that Luther protested as a Catholic within the Catholic Church: he sought re-formation of that which had suffered de-formation. He wanted his church to be truly and fully catholic and to take within itself again the pure Gospel. This the Church of his day rejected. If today, in the wake of Vatican II, Luther were to be received by the Church and his teaching fully integrated into it, there would be a conclusion and culmination of Luther’s protest: the Church would be truly catholic and evangelical.
Such words aptly summarize Luther’s desire for the reformation of the one, holy, catholic and apostolic church. His reformation has not succeeded, and indeed, cannot succeed whenever the gospel is compromised and church bodies cling to a narrowly defined confessional stance that ignores the full implications of the creedal assertions of the church catholic created and sustained by the apostolic message.
⁹ My thanks here to David Bagchi, who explores this in the paper, “The Problem of Catholicity in Early Reformation Germany,” given at the 2010 Society for Reformation Studies Conference at Westminster College,
Cambridge, England, April 7, 2010.
¹⁰ The immense popularity of this “dictionary” is shown by the fact that it went through at least 26 editions from various Rhineland presses between c. 1480 and 1505. Johannes Melber & Jodocus Eichmann, Vocabularius predicantium, sive Variloquus. The edition used was Vocabularius Jodoci doctoris et predicatoris sacre scripture ([Strasbourg: M. Schott, between 1484 and 1488]), Bodleian Library, Oxford (shelfmark Auct. 4 Q 5.1). The entries quoted appear at sig. Cviir.
¹¹ “Catholicus” is defined as “ein christenlicher mensch”; “Catholici: gemein christen menschen”; and “Catholica ecclesia: die gemein cristenlich kirch.” Melber & Eichmann, Vocabularius predicantium.
¹² This preacher’s manual was first published in 1502. The edition used here is Johann Ulrich Surgant, Manuale Curatorum predicandi prebens modum: tam latino quam vulgari sermone practice illuminatum: cum certis alijs ad curam
animarum pertinentibus: omnibus curatis tam conducibile quam salubre (Basel, 1508).
¹³ Surgant, Manuale, fol. 80vf.
¹⁴ For example, the 1520 treatise, “Fourteen Consolations,” LW 42:162–163; WA 6:132–133.
On Luther's understanding of "catholic", the article shows that:
[e]ach of these [documents] reveal that far from being against the one, holy, catholic and apostolic church as confessed in the Apostles’ and Nicene Creeds, Luther sought to recover the apostolic message of the gospel as a condition of the church’s catholicity. Apart from the apostolic message, the people of God cannot be catholic.
The question asked:
Was the Apostles Creed something that the Reformers tried to reform?
Answer: Yes. Together with the other 2 main Protestant traditions all three are united to clarify "catholic" to mean the community of true Christians of all time and place who believe the apostolic message of the gospel instead of "catholic" to mean the institutional structure of the Roman Catholic Church. It's just each tradition does the clarification in their own way: the Lutheran churches replace the word with 'christian' while the Reformed and the Methodist churches retain the word 'catholic' but add footnotes.