In AD&D and earlier, thief skills were not listed as having a skill modifier of "+x" but as "+xx%.
Simply said, every +1 is equal to +5%.
However, an important difference is, that in AD&D and before, there were no variable DCs that would have to be reached. Having a skill at 75% meant that the first 75 numbers out of possible 100 would be enough to be successful. If your skill was just 20%, then the first 20 numbers would mean success, but the following 80 numbers would be a failure.
A super quick and simple conversion would be to take a percentage, increase it to the next value ending in a 5 or 0, and then dividing the result by 5. (42% = 45% = +9; 17% = 20% = +4, ...)
However, in Pathfinder the DC is not always 20, so it would be useful to know what exactly you want to convert for what purpose, to maybe make some adjustments. Also, the abilities of thieves to Climb and Move Silently were not exactly the same things as the skills of the same name in Pathfinder. Everyone could try to climb a tree or move without being heard in AD&D, with no special rules on how successful they were at that. The GM would simply decide by himself if a wall could be climbed or if a group of sneaky PCs would be heard by guards.
But when a thief made his Climb or Move Silently check, he could do the seemingly impossible. Then he could climb walls that would be normally unclimbable for anyone but some other thieves, or move so quietly that it was impossible to hear him, regardless of how vigilant a creature was and how good it's ears. So it could be argued that the success of a thief skill isn't the same as making a DC20 check, which almost everyone might possible succeed at, but more like a DC 30 check, which would just not be possible for anyone but a rogue with high Dex and a lot of skill ranks. So adding another +5 to the conversion calculation I described above might be a good thing.
