Usually fare rules that require/allow historical fares mean that the fare is, for most intents and purposes, not really re-calculated (if they used current fares, it would be). As long as the new itinerary (and assuming this is your return segment) has the same booking code/inventory availability, there shouldn’t be a fare difference (maybe a penalty fee, which is a fixed amount). If your fare, say was only valid Tuesday-Thursday and you want to come back on Friday, they would need to look up a historical fare that was valid for Friday, although usually that’s not a big difference (eg, could be £20-50).

Oops, sorry, missed that the crux of your question was about the change from PHX to SAN. That’s harder to say without more details, but you can look up historical fares on Expertflyer. And certainly not as straightforward as a simple date change, but unlikely to be prohibitive in economy or even PE unless the flight is pretty full.