What happens to someone who bargains with a fae and exchanges memories? If you traded away enough memories, are you the same person? Is your soul the same without those memories?
That's the kind of question that has kept philosophers employed for a couple of millennia. Ship of Theseus-style stuff. If you slowly replace every board, nail, sail, and rope, one by one, over the course of a decade—every single piece no longer the original—is it still the same ship? Plutarch didn't have an answer. Thomas Hobbes asked two thousand years later: what if you took all of those original parts and built an entirely new ship out of them? Is that the Ship of Theseus now? Or is it something different?
I would contend that, for the purposes of Soulgazes, yes, there would eventually be a point at which enough memories are lost or altered to trigger a new one. When Susan did it, she lost about a year's worth of memories with Harry (but only Harry), so she's still the same person. I'd argue that, if Lea had gone all the way back to some foundational cornerstone memories (childhood, adolescence, first loves, etc.), then yeah, Susan would've been different enough that she'd be a different person.
On the other hand, the way Lea formed that particular bargain didn't actually erase the memories, she just kind of altered them to eliminate one element, and it created all kinds of logical inconsistencies. I don't know if Faeries can actually remove them altogether, even with a bargain. If they can't be removed, then I'd argue against my previous point.
