For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich. 2 Cor 8:9
What is 'rich' meaning? Making this about material possessions presumes much and ignores context.
The Q approaches from an odd stance regarding Jesus supposed pre-existence - as what we do not know. The scriptures tell us nothing about a Jesus who existed before he was conceived/born (bar a few muddled proof-texts). There is no Son except in prophecy - pointing to a future saviour - Mary's birthing of Jesus, the logos became flesh.
So how was he rich?
Jesus, in order to be the Lamb of God, sent to take away sin, had to be without sin from birth - holy - the unblemished lamb chosen from the flock.
“The Holy Spirit will come upon you, and the power of the Most High will overshadow you; for that reason also the holy Child will be called the Son of God. Luke 1:35
Jesus managed to maintain this sinless state to the bitter, yet triumphant, end on the cross. Not on his own of course. He was made like us in every way and needed support and all spiritual provision from his God and Father - just as we do. Mk 9:29, Heb 5:7, John 5:30
Therefore he had to be made like his brothers in every respect... Heb 2:17
In every respect except being a sinner. It is this reality from which his richness is derived. He was born in the image, the form of God, having God's status of being without corruption and the infection of sin. He could call on legions of angels, or many other marvelous benefits as the beloved son of the Father - He would supply all Jesus needed.
But Jesus had been sent - to choose another path - to do the will of God, even to an horrific death.
Jesus explained, “My food is to do the will of Him who sent Me and to finish His work. John 4:34
There were many occasions where it is noted Jesus had to submit his will to God's will - 'thy will be done'
This is how he emptied himself - to put aside his privileges, his self-centered will which would have allowed pride to fester and end the mission of him being the Lamb for all creation.
His cast-aside riches became our riches - through his voluntarily giving them up. What are our riches? To live as God does, to know good and do it, to know evil and resist it - just as Jesus did his whole life.
...but emptied Himself, having taken the form of a servant, having been made in the likeness of men. Phil 2:7
(How some consider Jesus being somehow God and still made like us in every way is a mystery not inspired by the text)
As a superior sinless man with every imaginable benefit as God's son - rich in power and influence, he could not become the humble servant required for the sin sacrifice. Indeed, he was not ready to be the sacrifice until he had learned what it was to be obedient. This is not an arbitrary, 'ok, you understand what obedience means, you are ready', from the Father.
Although He was a Son, He learned obedience from the things which He suffered. Heb 5:8
Jesus' poverty, humility, willingness to lay down his life, enabled him to be the perfect Lamb - gained by obedience learned from suffering.
Unless he fully trusted the Father's provisions of Spiritual guidance, wisdom, love etc, he would have faltered and failed. He could not trust in himself, his way or his will. Jesus' own power and influence would be the undoing of God's mission - to defeat evil by the power of love through this holy man, the last Adam. Jesus still had access to these 'riches', but he determined not to use them (tempted though he was) - not to put his will before God's and seek an apparent better way of doing his Father's work.
We see this, 'although he was a son' (Heb 5:8) - of God in the fullest sense - all God's fullness dwelt in him. Yet this privilege of son-ship was set aside - becoming poor and as a 'suffering' and humble servant to his Father and his brothers. Only now can he fulfill the mission of accomplishing his Father's purpose - not by his own merit or power or wisdom, but of God's in him.
This is grace!
For you know the grace our Lord Jesus Christ, that being rich, for the sake of you He became poor...
The grace of God lived out in God's logos, His will and plan of salvation - Jesus. We cannot define grace merely as 'unmerited pardon'. It is far grander than that. Jesus put aside his riches to enable a true servant to emerge. He was called to serve his God and all of us.
Indeed the 'he became poor' is a process reaching it's climax in the garden - 'not my will but yours'. The last heroic, courageous and humble act in service to the Father and us. (Luke 22:42)
For even the Son of Man came not to be served but to serve others and to give his life as a ransom for many. Matt 20:28
Paul begins the passage with,
that in much proof of tribulation, the abundance of their joy and of their deep poverty abounded into the riches of their generosity.
They too experienced this cycle of having much, but came to know the deepest loss can be turned to good...
that also you should abound in this grace.v7
Notice how Paul always starts his letter with, "by the grace of God", "the grace of the Lord Jesus". This is the key to unwrapping what Jesus has accomplished and how he did it. And how, being made one with him, we will do it too! The highs and lows, the emptying and the filling in God's tender loving care and wisdom.
We can only grasp the concept of, 'though he was rich' in the context of, 'you by his poverty might become rich'.
In achieving the task of being the perfect Lamb, we now can indeed be the brothers for eternity - becoming rich, as he was and now is again, seated with him (Eph 2:6) at God's right side, ruling over all things having inherited all things. Heb 1:2 And we also having inherited the same - his life, his glory, his possessions. These he was granted by God, and he shares them with us.
For all who are being led by the Spirit of God, these are sons and daughters of God. ...and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with Him so that we may also be glorified with Him. Rom 8:14-17