Hold on, lemme think about this a second. (I do this all the time.)

Okay.

You have word, appleworks, whatever, SOMETHING that lets you find/replace all, right?

Search the source code for the tag < i >.
Replace with < omg > or something else meaningless.
Search the source code for the tag < /i >.
Replace with < i >.
Search for the source code < omg >.
Replace with < /i >.

That will switch all the tags around. You'll only have to hand-edit the one at the beginning and the one at the end.