I was actually thinking of a complete override, but now that you bring it up... perhaps allowing their 'original' memories to gradually return in pieces might be an interesting idea for those willing to put the work into creating their AU persona.

The canon identity would remain dominant and the original memories would be in the background, but I'm amused by the idea of a male omega suddenly realising they've given birth to fifteen kids and loudly freaking out about it because WHY?!

(It's omegaverse. Mpreg is part of the whole concept, and I'm still considering ways I could handle it/avoid handling it).