That is a definite possibility, the CR's are not set.But why not stay closer to 3e, 4e and 5e CRs, and then let the number of mythic stages be the upscaling factor, like you considered in an earlier post?
However, I am not going to be slave to tradition. It is not like these things are consistent anyways. I mean Demogorgon had 3 different CRs in 3e/3.5e! BoVD = CR 30; Fiendish Codex 1 = CR 23; Dragon 357 = CR 33. That is a 10 point spread! By that token I could make a CR 37 version of Demogorgon for 5e.
Anyway, I've got to do what I find interesting, fun, and just feels right to me, or it just will not happen. That, to me, is more important than an adherence to an arbitrary history.
Why subject 5? 4e Demogorgon at lvl 34 is pretty close to the only really epic Demogorgon in 3.5e at CR 33. And this are "epic" updates? I actually based my original epic update CR off the 4e lvls. Regardless I will revisit all of this when I get back to it in a month or so.Edit: myself I'm thinking a base CR of the 4e CR minus 5 (except Gods) . So Demogorgon CR29, Tiamat CR30,Orcus CR28, Ancient Red Dragon CR25, Lolth CR34 and so on (minor adjustment for some monsters like the Tarrasque).