Yes, exploding them (to their original layers), and then I would use Mask/Color if there are a lot of entities of more than one color, to change to the colors that you need them to be. And then Redefine the old symbol to the new version -- and then you can use Reload to change all the instances of that symbol to the new color(s).
Thanks much, Neil. Being an old acad user (with a long hiatus from CAD) I seem to remember that one could define blocks with colors and linetypes by layer or some such and thus have some control over block colors without redefinition. Doubtless on someone's wishlist.
