Have they shared this at all, yet? I'm currently working on a project converting monsters for other people to use so I can't wing it for what works in my game. As I'm going through and checking monsters that Paizo has already converted I'm noticing things like the fact that monsters no longer have a full attack option. This has caused changes like
Axebeak 3.5
Attack: Claw +4 melee (1d6+3)
Full Attack: 2 claws +4 melee (1d6+3) and bite +2 melee (2d6+1)
to
Axebeak Pathfinder
Melee: bite +6 (1d8+6)
so . . . Are they simply using the groundrules that they've set up for monster creation or was there another set of guidelines for the conversions?
A mixture of this conversion guide and the appendices in the back of the Bestiary, starting on page 290 should really be all you need.
Some monsters might have dramatic differences if they were considered to be poorly built for their CR, but for the most case those two above resources should cover all the basics.
In some cases they are also going back to the 1e versions of the creatures for inspiration. I believe the 1e axebeak only had a bite attack.
And as V-P posted, some changes are to bring the monster into better alignment with their CR rating (the dragons are a good example of this).
Gorbacz - From what I'm seeing looking through my books the Attack/Full Attack started with 3.5. I'm guessing by "later 3.5" you're talking about MMs 4 and 5 when they screwed up my stat block :p (Yes, it's mine, I stole it).
Thanks for the input, all, I'm just finding myself having to flip a lot of pages, I'm sure I'll be genius at it soon.