It depends if we want to keep the information intact of the older blog posts. Since we are migrating them, I guess the answer is yes so why would we apply a lossy transformation?

To clean things up and make things more consistent. I'd rather work with a clean list of tags moving forward. One could even imagine some sort of verification step when trying to use a tag which does not exist. IMO several of the existing tags are useless and others can as suggested be merged.