They would have to rebuild the game from the ground up.
Look at all the bugs, minidumps, desyncs, etc. they deal with now. Moving all that code over opens the whole can of worms back up. Sins code seems to chain react pretty easily--one thing tips three others or breaks it all, etc.
To do a through job and really proof it would probably require about the same amount of time as just completely rewriting it--so you are talking about developing an old game instead of making new ones.
Since Rebellion has already taken a lot of time, I can't imagine that's possible now (not to mention they already said it isn't).
I'd hope that Sins II will get made one day in the NOT too distant future and they might add things in like stock markets, more setup and map design options, more customizable features, etc. to really broaden the game out--not just dub it over as is.
But for now, here we are at 32 bit. Mods are the answer to your texture questions. Using a TSOp-lioke approach where you cut out higher resolutions carefully and redundant and over-sized textures, you can probably squeeze a few more in.