Need some advice....

I am scheduled to fly this Saturday from PHL>MEX via PHX. I am on the 7:05am PHX filght, getting in to PHX at 9:08 am, then the flight to MEX departs at 9:56am (tight connection). As of now, I am in coach for the PHX flight and F for the MEX flight. The MEX flight gets in to MEX at 3:04 pm (I am going to a concert that night at 7pm). This morning, the PHX flight is showing as sold out on US's website, and seatcounter is showing it zeroed out.

Add to the mix the weather forecast of rain on Saturday morning in Philly along with my personal history of delayed flights into PHX from PHL, and I am worried about missing the connecting flight. There is only one other flight out of PHX direct to MEX on Saturday, but it leaves about 20 minutes after the US flight.

What do you think my chances are of going to the ticket counter on Saturday morning and having them change my flight to go via CLT on the 6:45am flight? That will get me into MEX 3 hours earlier. Think I'll have any luck consdering the (hopefully) oversold PHX flight?