My 2 cents, and a word of experience:
DO NOT use RetroGRP for such purpose. ITtcan get buggy/crashy. OPen the MPQ manually (WinMPQ, CV5, whatever), extract the grps you want...and THEN use RetroGRP to deal with them. That's the best and safest way to alter SC graphics, according to my experience.
In case you don't know which GRP files to look for, check out this:
http://www.stormcoast-fortress.net/Spc/spc3b1.htmlAnd use the
GRP ID/Name column for identification. I Hope this helps...
