I don't think angels can be promoted from one order to another within the 9 orders. Simply because they are in that specific order by their very nature. Just like a dog can't be a cat or a Rose can never be Lilly.
Also note that they are helping us to save our souls because God wanted them to do that, not to get "promotion" in their Job. After the end of their duty, the angelic orders will continue to exist, but their offices will not be altogether the same as they now are, for they will then no longer need to help human beings in the process of saving their souls.
The distinction of grades among the angels ... will ever remain in the angels; for these differences of natures cannot be taken from them unless they themselves be corrupted. The difference of glory will also ever remain in them according to the difference of preceding merit. As to the execution of the angelic offices ... It will cease accordingly as their offices are directed towards leading others to their end; but it will remain, accordingly as it agrees with the attainment of the end. Summa Theologica 1:108:7
As said above by St. Thomas Aquinas the only purpose of Guardian angel is to save our souls, their function towards us cease to exist after we enter heaven or hell. (Since we are in the process of being cleansed, our angels do continue to pray for us and urge saints and humans to pray for us when we are in purgatory)
- Our Guardian Angels are not our servants but our masters.src
- In heaven we will have God and so we will lack nothing. He will be our only need and our provider of our need, we will not need an angel's help anymore.