Just like in all PSX/PS2 (and probably PSP) games, it's matter only of file/folder names (of course until save format from both region is the same, if not no one than developers of this game could help with it ;] - in other words: this will not work on all games).
I've successfully do this with "Uncharted 2: Among Thieves" and "Uncharted 2: Among Thieves - Game of The Year Edition" (Greatest Hits edition).
1. Rename save folders - in this case - folder with save profile and folder with game progress:
from "BCES00727_NDI_UNCHARTED2_BT_2" to "BCUS98123_NDI_UNCHARTED2_BT_0"
from "BCES00727_NDI_UNCHARTED2_BT_P" to "BCUS98123_NDI_UNCHARTED2_BT_P"
As You can see, save slot it doesn't matter.
2. Replace PARAM.PFD and PARAM.SFO from new save to old save.
3. Run game and check if he regonize save. Play for a while to overwite save's (because in other case *.sfo will be not updated and in XMB show false info about save).
Of course save owner should be the same. If not, You must fix user accounts id, user number ids and ps3 id (if this is possible, changing IDs not work on all games).