Well if you look at most armies basic troops, the models themselves are practically the same, they have just been updated to include extra parts, for example CSM have pretty much the same models, just a couple extra spikes added to the special weapons and teh addition of a standard bearer.
The Imperial guard I think have aged fairly well (or at least the plastic ones have) if there are going to be new ones I would expect plastic variants of vostroyans and steel legions etc. (the versions still in metal).
If its plastic already its unlikely to be updated. If its metal then they will almost definitely release a plastic version now if there isn't already a plastic alternative to fill the gap. SoB will get a plastic range if they ever release a full codex, its more whether there will ever be such an update.
Me: To be honest im amazed there isn't a chaos god of not revising or at least chaos god of procrastination
MidnightSun: There will be, when enough people do it. Y'know when the Eldar were all engaging in their hedonism and Slaanesh ripped open the Eye of Terror with his creation? Slaanesh will have NOTHING on the God of Procrastination, and his vile minions will carve the galaxy asunder in a thousand year reign of blood.... Tomorrow