There currently isn't any tag management features in Clearspace that would allow you to accomplish this. The reasoning so far has been that tags are meant to be freeform and little used tags will drop off the tag cloud and never be seen.
There is a way to accomplish what you want but it'll require executing some SQL directly against the database (with Clearspace shutdown).
"UPDATE jiveTag SET tagname='yournewtagname' WHERE tagname='theoldtagname' "
Let me know if that works for you.