Rebuilding the Bashed Patch with the changed tags is fine to do prior to reloading a savegame in most cases; however, there are always potential anomalies with savegames, since they contain all of the world game data at that instant.
That said, It is 'technically' best to perform a clean save (see the Troubleshooting Guide). I almost never perform a clean save unless I see something weird happening with my game after a mod change. I can't recall ever having an issue like this when updating a Bashed patch, but it is possible I guess.
In this case, it may be that a clean save is warranted or there could be a delay in the result, but I personally' would not do any of that prior to having some reason to do so, since this it is just speculation. Nevertheless, I'd recommend that you follow the clean save procedure if you are anxious to just play the game rather than tweak away.
Or post your question on the latest WB dev thread and get back to us with the response. You want to pose the question like: Do adding delev/relev tags to a plugin via the Bashed Patch have potential issues with savegames associated with Bashed Patch prior to adding these tags?