on the sentra forums, a few guys got a dealer to reflash their ecu. i called a dealer, and they said no way would they do it. from what i read, it seems to give around the same gain as a tune if your n/a, which i am. now, where could i reflash, and if i get one, would my safc-2 be of any use? its not connected, its just installed. im just worried if i pay a dealer for a reflash, it actually gets done. dealers don’t have dynos, so i couldn’t really see the gain until i found one. by the way, i have the qr25 engine(spec-v, se-r)
call hybrid.
can they relash ecu’s? i hope so, i love that place. they hooked me up on my header+install. and it was great work. 
it depends. What is this reflash for? Is it some type of Nismo related flash? I think you need to find more out then what you seem to know.
A tune is basicly the same thing, only you may have the ability to make the reflash/tune talered to your car and aftermarket parts on your car.
Gratned i dont know whats out there for the sentra world, but most of this is pretty basic ideas.
if interested I have a friend who builds megasquirt efi controller:http://www.bgsoflex.com/megasquirt.html
they are fully programable dyi stand alone system pm me if you are interested