@mitra: Good point, yes, Serif will replace Bookerly in that case.
The good news is it should then be trivial to replace Caecilia with something else. There's an example config that switches Caecilia for Bookerly, for instance.
(Do you already see both Bookerly and Caecilia in the Aa menu on 5.6.5? Can't remember, but that's the case on >= 5.7).
I must have done it that way for a reason, because right now that seems like a weird workaround instead of simply just overriding Caecilia from the get go, but, hey...
.
Note that on >= 5.7.2, you also get an extra font in the Aa menu (Open Dyslexic), which makes another perfect candidate for replacement
.