Twist =/= retcon, necessarily.
And assuming this was the intended twist, Bioware would have wanted the playerbase to expect the worst in Flemeth's intentions, no? Anyway, I didn't feel like this was a retcon. I never read the grimoire or saw what was written, I simply took Morrigan's word for it, believed what Morrigan assumed to be true. Morrigan and Flemeth have trust issues, I think. Issues that are not unfounded, whether Flemythal intended to forcefully posses her daughter or not. XD
Exactly, when you actually look at her actions in Origins and DA2, we never actually see Flemeth do anything but help the heroes (although it's self-serving help), nor do anything that could be construed as evil? The only time she becomes hostile is after the Warden has refused her offer to peacefully take the book and leave, so she was well within her rights to defend herself?
Morrigan makes mention of her murdering her past lovers, but we cannot take that at face value because Morrigan (as much as I love her character) has taken liberties with the truth from time to time? It could be that she plied us with those stories because it helped to establish her mother as a malevolent old witch, which would increase her chances of getting us to agree to murder her? I doubt that she wasn't planning that even before we found the Grimoire in the Tower, but this newfound discovery of a possession ritual only hastened the urgency?
Same with the Grimoire, we never read the tome (because it's in a language only Morrigan conveniently understands) so we have no idea what it actually said? For all we know, the ritual instructions could have been in there, but the description was super vague and said something along the lines of "a ritual to facilitate the transfer of one soul into another", leading her to misinterpret it as being a form of hostile possession, rather than the benign "gift" that's apparently meant to prime her for Mythal's soul in the future?
While it might be a retcon, it could just be in keeping with Morrigan's characterisation over the series, of someone who while very intelligent, is not nearly as smart (or wise) as she likes to think she is? We see this clearly in her obvious irritation with Solas for correcting her about the Elven Gods, as well as Flemeth's own summation of her daughter in DA2, that "she's a girl who thinks she knows what is what better than I, or anyone."