Advanced Monster Caster Level

[Diirk]

Well it doesn't mention it, sort of:
"Make sure to “scale” your evaluation of these abilities by the monster’s current CR."

Which to me implies that any special abilities the monster may have should be improved to stay relevant to its target CR, if necessary. Advancing a nymph from 6 hitdice to 10 hitdice only raises its CR by 1 (if you're going solely by its type), so obviously a +4 caster level increase is much too powerful, however simply adding 4 hitdice and not touching its casting abilities goes against the spirit of such a creature.

Of course you could always just advance it by class instead.. give it a few druid levels, and the non-elite stat array. Easy way to improve its caster level, as they both stack

 

[Patryn of Elvenshae]

Well it doesn't mention it, sort of:
"Make sure to “scale” your evaluation of these abilities by the monster’s current CR."

My reading of that line is that, for example, a CR 2 creature who can cast Sleep 3x per day as a spell-like ability at CL 1 would really be CR 1 without it. Therefore, his SL ability has affected his CR.

As you advance them a few Hit Dice, that ability becomes less and less useful. Once he's been advanced enough to qualify for CR 5, that Sleep ability is largely useless, as the parties he'll be facing routinely will be immune. Therefore, that 1 CR bump is no longer appropriate.

Likewise, a couatl casts spells as a 9th-level Sorceror, out of the book. As a CR 10 creature with 9 HD, that's pretty potent. He makes a good boss creature against which to pit APL 7+ parties. However, when advanced to a 27 HD creature, those spells are much less potent when compared to, say, the protections available to a 15th-level party. Therefore, they aren't worth as much, and maybe his CR is, at this point, slightly overstated.

 

[Diirk]

Well sure, it can mean that too. However its in the section about adding special abilities so maybe its just me, but I'd prefer to update a monsters flavour rather than just turning it into a generic punching bag as you 'advance' it. Spell casting monsters that get additional HD but no better casting ? Meh, so bland. Much better to give them less hitdice but to advance their special abilities as well. Of course, such things are an art more than a science to coming up with exact rules isn't really possible

 

[Abstraction]

It might be important to note that if advancing a creature's HD results in higher base stats (Str and Con), then abilities based on those do improve.