Advanced monsters and their powers

Hi all,

I have been advancing several creatures to challenge my high level party (they should be around level 15).

Two creatures of concern are the Spirit Naga and the Cloaker.

I have tripled their HD to the maximum, and adjusted the DC's for their special abilities (10+1/2HD+Con bonus for the poison, and 10+1/2HD+Wis bonus for the charming gaze for the Naga).

However, the Spirit Naga also casts spells as a 7th level sorcerer (which is two levels below it's HD, which is the same for the Dark [7th]and Guardian [9th] Nagas). Only the Water Naga has a caster level equal to its HD.

So, my question is, having advanced the Spirit Naga to 27HD from 9HD (increasing its effective CR from 9 to 13), should I also advance its spellcasting powers to match? Should I follow the formula that seems to indicate a sorcerer level 2 levels below its HD (in this case, 25), or else should I triple it as I did with the HD (ie 21)?

Neither of these seem to fit, as a 25th or 21st level sorcerer has a higher CR than the advanced Spirit Naga (25 or 21 vs effetcive 13), but on the other hand I would think that along with an increase of the Naga's size would come with some increase of its sorcerous abilities.

I just don't know what the magnitude of this increase should be (if any). Does anybody have any ideas on this subject?

Thanks for your help!

PS - similarly for the cloaker; its Dancing and Silent Image powers are cast as if by a 6th level sorcer (equal to its HD). Should that be tripled as I did with its HD (this will affect the number of images created by the dancing images, and the durations for both powers)? Or just left at 6th or somewhere in between...?

According to the rules, their abilities don't change. The MM2 had creatures like that where their abilities got more powerful (the spellweaver advances via Hit Dice, and it has sorc levels = HD +2... advancing it is broken!)

In the case of the naga, you should give it sorcerer levels (they stack).

In the case of the cloaker, the offensive abilities have save DCs that increase, whereas the defensive abilities don't need to last any longer (a combat should last about 5 rounds). They can't be dispelled (AFAIK), so there's no need to raise the caster level.