OK, but there's still only one other E7W flight out of TPA that day, and it's too late to make the last flight to LAS. So my advice remains the same, except to state that (a) the E7W (E-175) fleet is about 3x the size of the E70 fleet, and (b) the E70s are all operated by Republic, whereas OP's current flight is operated by Mesa. So, it would take a fairly significant ops disruption before UA would consider that particular swap on the day of departure. It seems much more likely that any E7W -> E70 downgauge would happen 2-3 days before the flight, at which point, OP would likely be moved automatically to another flight. Still not something that I'd pay a change fee to avoid.