Even if they switch to a new engine, they still have the code from the previous iteration that they'll have to work into the new branch at some point... and as we've seen already numerous times from CIG, whenever they've iterated onto a new version of code, the old bugs from before either get coded out (yay!), stick around (oh no!) or, most worryingly, evolve into entirely NEW bugs (holy ****!) that cause all manner of problems for the coders to resolve.
Game just looks like is very over complicated already and engine switch to LY (even its only a moded CE version) complicates everything even more...and also lets don't forget that LY is only in Beta state, so good luck fixing all the bugs CIG...so I do not expect anything more or less bugfree and stable running at 60 fps anytime soon..