Regarding the conversion of previous editions (even 3.5) to Pathfinder, here's my experience:
The core rules in Pathfinder have changed. I was running a Pathfinder WotBS game, and ran into more than one situation where the players had abilities, stats and wealth for their level in Pathfinder, but then I was making them face the creatures of the encounters "converted to remain true to the AP".
The problem with this is there's a number of things that have changed about creatures in Pathfinder. For one, they don't normally advance creatures with Hit Dice, and instead like to apply templates. Second, there's a number of creatures that were changed to be done in a much simpler way (using templates) or had their CR changed to match their abilities.
Then, there were a number of creatures that were simply not converted (yet).
After having a bunch of encounters that didn't really fit well, I decided to drop the "cleave true to the AP" and built encounters around the
Stats by CR chart, and
Party level vs Encounter Level chart, as it's depicted in Pathfinder.
If the players are playing by Pathfinder rules, then I need to challenge them with Pathfinder rules.
If this meant advancing Ghast stats, or adding a couple extra creatures for an encounter, or writing up a Hieracosphinx's stats from scratch, using the CR chart to determine what it should have for the CR encounter it's supposed to be.. well, so be it.
It won't be the same gameplay experience that it would have in 3.5e.. but honestly, that went out the window when the players made Pathfinder characters.
The party ends up being properly challenged (no unexpected "too hard" or "too easy" encounters), and everyone has a good time.
A lot of work on my part as DM though...