Automatic changing of tags can be very problematic. I think that re-tagging really needs to be a manual effort because you have to read the post to figure out what the tags should be.
However, the re-tagging process needs some serious streamlining. I occassionally try to do some re-tagging because I have slight OCD when it comes to organization of things, but it becomes incredibly tedious in a short amount of time.
What we need is system that will support re-tagging in batches:
Step 1: Select Source tag
Step 2: Select Target tag
Step 3: System shows you posts with the Source tag one at a time. For each post you say "Source", "Target", "Other", or "Remove" to choose whether to leave the Source tag alone, convert it to the Target tag, convert it to some other tag, or remove the tag entirely.
This would make it very easy to convert tags but allow manual confirmation that the tag should be converted. I don't think that it's possible to fully automate this process but the process described above would make things as easy as possible to correct.