MCCORMICK

Ecm went bad and I bought a used one from a guy thinking that any or all ecms can be used. My truck is a 2001 6 speed and when I went to the dealer to have them flash the new one they're saying it's from a 1998. Stupid me I didnt ask what year truck it came off of when I bought it. So the technican tells me I need a 2000 or newer ecm and that there's no need to flash anything it will work. I've spent alot of time reading about this issue and that's the first I've heard that. So I guess my question is what years work with what and does auto/manual tranny make a difference. Thanks for any help.