I don't see much reason to divorce the processes. I'm sure there is probably a separate technical term like Metamorphosis to describe the process by which a Unicorn would be physically transformed into an Alicorn, with both Wings and a Horn.
However, all that process would get you is a shitty fake-princess that annoys the fans. So I'll just connect it with the other half of the process to make a real Alicorn and refer to the entire thing as the single process of Apotheosis. The Ascent to Godhood. Could also be called Deification, but that just doesn't sound as cool.
This picture sums it up pretty well. Of course, if Sweetie ever managed to acquire the power of even a Hero Deity with a Divine Rank of 0, she'd probably be 20th level or working her way into the Epics. So of course, she'd already have an appropriate Cutie Mark. Something like a lemniscate, or a spiral galaxy collapsing into a black hole.