Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7D11 Safari/528.16)

I think you need to ask yourself if saving $93 is worth the effort and potential high price of a full Y fare. I'd just change it and keep an eye on the prices. You can always refare it.

It won't matter if a phone agent notes your PNR. They'll cancel your reservation when you no show.

If you want to try it by all means go for it but I think you'll be in for a nasty, expensive surprise in Vegas.