Singapore 777 -
I'm no techie, but based on what I know... the different 772s have different weights and configurations (fuel tanks, etc), among other things. (Ejazz - can you confirm/clarify?). I don't think it is a simple case of switching engines - a few other things will also have to be tweaked. Also, AFAIK the FAA (whose directives the CAAS follows) requires different certifications for different configs, so this new engine+config combination will need new certification.
You may get a better answer in the tech forum.
In any case, this discussion is moot anyway. I don't think the three-class SQ_ series aircraft have crew rest facilities, and they aren't fitted with SkySuites/SpaceBeds either.