On my last A320 flight, there were two tail swaps within 36 hr of departure (149 pmVX config to 150 AS config, then finally to 146 pmVX). I noticed they had already processed premium and first class upgrades at the time of the second tail swap, although I only saw 7 seats occupied in F, which led me to wonder whether they hold back on F upgrades until they know for sure the assigned a/c will have 12 F? Not sure what they do for premium class upgrades when the plane switches back to the non-reconfigured version with fewer premium seats (probably easiest to just leave everyone in the same seats than to try to reseat everyone).

In both cases where I saw tail swaps close to departure it was on 1208/1213.