3.0 to Pathfinder what would be the issues?

[Elder_Basilisk]

A little bit of an odd situation here. One of the guys in my group (we trade DMing duties) has been DMing Return to the Temple of Elemental Evil using the Pathfinder rule system. I was talking to him today and he's finding converting to Pathfinder a difficult chore. (I know). I suggested a few things but what would be the big difficulties if he didn't convert at all and just ran all the 3.0 stat locks using Pathfinder ruleset?

For me, the big things would be:
1. Lack of CMB/CMD into. (Our party doesn't use a lot of combat maneuvers though. Quick solution--just use attack bonus if any of them try a combat manuevers. If you need CMD on the fly, just calculate it quickly as HD plus str plus dex +size)
2. Don't gimp the class leveled goes. Pathfinder gives a few new abilities and more feats.
A. For clerics, they are ahead feats at level 5, 7 and 11 and 13. Quick solution--just change turn undead to channel and assume they spent the feat on heavy armor proficiency. Give every level 11+ character an extra 2 HP/hd for toughness and favored class. Done.
B. For leveled monsters, give them +2 HP per HD and call it good. Assuming they have a favored class bonus and took toughness it should be close.
C. For fighters just give them all +1 to hit and damage across the board for weapon mastery.

Use Pathfinder version of any spells.

I think that should be a very playable conversion rubric that doesn't require any prep or rebuilding. Am I missing anything?

[RandomPeasant]

I would just do the thing plenty of people already do and declare that you are playing "3.PF", then use stat blocks as-is, and let people play PF classes or take PF feats. There are a ton of fiddly bits in stat blocks that PF wants you to change, and that is honestly a huge pain in the ass to do for something that is probably never going to matter.

[Scots Dragon]

The two systems work on the same maths, just run things as if the party's a level past where they actually are and it should be fine.

[Kurald Galain]

I think that should be a very playable conversion rubric that doesn't require any prep or rebuilding. Am I missing anything?

There pretty much is.

Almost all D&D monsters also exist in the PF Bestiary, so just look up their stat blocks at d20pfsrd.com.
For humanoids with class levels, go to this page (https://www.d20pfsrd.com/bestiary/npc-s/) and grab the closest equivalent. Why yes, those are hundreds of pre-built statblocks. Like, if the scenario calls for a CR 9 evil priest, go to that list and grab the Battle Oracle or the Plague Cleric and just use that. Maybe swap a spell or two if the tactics really call for it, and you're all set.

[Akal Saris]

I ran some 3.5 modules in PF in the past, and there's a near-infinite amount of little fiddly things that you could touch if you cared, but pretty much everything can be resolved quite easily if you're willing to apply simple modifiers on the spot:
- For monsters, I gave everything +2 to-hit/dmg if it's part of a group, and +4 if it's solo. Technically their HP should go up as well, but let's be real here: almost nobody wants extra combat rounds. Better to make the monsters more dangerous than make combat take forever.
- For any type of skills type challenge, feel free to add +2 to it on the spot if it seems too easy for the players (which, if I know my early 3E modules, is almost all the time). PF hands out fiddly little bonuses to everything compared with 3.0.
- For CM bonus/defense, use their BAB, add 2-4 depending on how good you'd ballpark the monster at something

I suspect your DM is trying to keep things 'by the book', which is commendable, but he can probably save himself a lot of time and effort if he changes his approach to applying some quick modifiers while ignoring the 'actual' stat blocks. It's going to lead to the same ideal result (a fun and challenging game for the players) with much less work.