I really wouldn't do that, it's inconsistent with other stock XenForo behaviour:
- If you change the destination forum for reported posts, posts reported prior to this change will stay in the previously configured forum
- If you change the author, title or message of the welcome converstion, existing welcome conversations will stay as they are
- and so on
If you really think such a feaure is necessary, I'd implement it as as a cache rebuild as already suggested.