Is this SRAM X-9 mech 10-speed compatible? I have this SRAM X-9 mech on my Dahon folding bike. I bought it second-hand, so I'm unsure of it's age. The bike is fitted with 9-speed shifters and cassette. I would like to upgrade to a 10-speed system, and wondered if this mech will handle 10-speed or if I'll have to replace it?
To quote a SRAM representative on the question of using 9 speed derailleur with 10 speed shifters:

MORGAN @ SRAM: For the rear, no. The cable pull is different for the 10-speed rear derailleurs and the 9-speed rear derailleurs won’t line up properly with the cogs when used with a 10-speed shifter and cassette. A 9-speed front derailleur will work, but it’s not optimized and may result in more chain drops or slower shifting -or no shifting at the extreme ends of the cassette- because the gap between plates is larger than what’s required for a 10-speed chain. The cage is also shorter if you’re viewing it from the side because it doesn’t have to work with as large of a chainring as what’s typically found on a triple.

So, you'll have to replace it.