the Jester
Legend
...since swapping spells specifically does not change CR's for spellcasters
I didn't see that in that link to Mearls' Twitter, but I didn't follow it for long- did I miss it? Or is that stated somewhere else?
EDIT: I see you answered this above, thanks! That's advice I will discard, for sure.
I have always very much felt that changing spells does change a creature's CR; there's a vast difference in challenge between a wizard who is packing meteor swarm and one who is packing, say, astral spell. For that matter, you could- not saying it's a good idea or sensible, but you could- design an archmage's prepared spell list with nothing above 2nd level. That definitely ought to change the CR.