What happens to your legacy when you transfer characters in SWTOR?
Solution 1:
You'll have everything from your Legacy on the new server (unlocks, level, etc.) and the old should be unaffected.
It will combine things if you have different Legacies (i.e., a character already on the new server) with different unlocks so you get the best of both worlds, including the higher Legacy level.
For slightly more detail see Dulfy on Reddit.