You'll have everything from your Legacy on the new server (unlocks, etc.) and the old should be unaffected. It will combine things if you have different Legacies with different unlocks.

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.