Yes, it will send you to the bottom of the list pre-departure as the list is FIFO ordered. So by changing the request you'd be cancelling and then re-listing. If anyone had requested an upgrade on PA after your original request, then they would then be ahead of you.
If you're talking about at the airport, then the only thing you'd be sacrificing is the "time" tie-breaker on the DM.
So bottom-line, don't switch it unless you absolutely need to or are very positive you'll get the upgrade in any case.