2.6!? As in, circa February 2003? Wow, I don't think you've got much of an upgrade path left there. That will be a fun project.
You're right that a facelift is the easy win, but I don't see it as orthogonal since you've stated a version upgrade is in the forecast anyway and any skinning changes you make now will likely be obsolete after an upgrade/migration and will need to be redone. Since any modern blog package will preclude the vast majority of the CSS changes you're probably making by hand, you'd be killing two birds with one very large, very unwieldy stone.
This is all philosophical debate, of course. Talking shop. Thanks for humoring me. Besides, I bet all you'd have to do is comment out a substr() call somewhere to enable "full feeds." ;-)