I dunno. desoldering and replacing a large fine-pitch SMT part is not going to be very easy...
As for the Arduino, I think all the existing boards are using the "biggest" CPU with that particular pinout. There is no 28-pin AVR with more memory than the 328p, nor a 100pin chip with more memory than the 2560.
(though I WAS somewhat disappointed that the new "xmega" chips were not pin-compatible with the 1280/2560. But that's Atmel's fault!)