If I change Grand Company.. can I go to Frontlines?

Or do I have to rank up again before I can enter again?

Yes you can.

Source: 3 of my FC members did it to join me, but you start over in the new GC.

i switched to adder a few days before 2.3 and started doing frontlines before i was maxed rank, kind of sad too cause the twin adder seems to never win :{