I think we're free to head-canon it in whatever way makes best sense to us in our own playthroughs. Which might even differ from character to character.
Though I assume that it's true that we are limited in that head-canon to something that makes sense of all the tags that will have been set in our game. So even though we might tell a different story about who our character is, their motivations, etc if we choose a retcon-type head-canon, the things they have actually done in the game will remain the same, or at least anything that has consequences after the point of respeccing.
I can see a case for thinking of it this way, and I guess that is a fair characterisation of what I did when I respecced my WotR character, who I'd started off as a rogue then took levels in sorceror intending to make her an Arcane Trickster, who needed two levels of sneak attack as per Pathfinder rules. Once I took the Trickster mythic path and realised that would give her the two levels of sneak attack without taking rogue levels, I respecced her as a sorceror at level 1 and pretended that is what she'd always been. So yes, I definitely retconned my WotR character. And I'm not even sorry
