I was diagnosed with complete previa at 21 weeks. I moved and switched OBs at 24 weeks. Both OBs completely agreed that nothing can be confirmed until closer to 28 weeks and then even so things can continue to move away up until you deliver.
I freaked out after Googling, but now (27 weeks) I have heard so many good outcomes that I am not worried. Also, its incredibly misdiagnosed (this from my Aunt who a high risk u/s tech). Something like 5% of low lying placentas end up with previa.
So try not to worry!