I don't know if you can upgrade your Celeron M to Pentium M. Its very likely you can upgrade to PM, especially those 400MHz fsb cpus model ending with 7x5 with pin connections. People said some laptop manufacturer with some models use ball type connection, some with pin type and some with welded. You have to take it apart to find out if the cpu is welded to the mainboard.
Not necessary Celeron M runs hotter than Pentium M. Power consumption for Celeron M is 21W and Pentium M with FSB 533MHz is 28W. If your application runs the pentium M at peak performance all the time, then PM will be running hotter.
My friend has an Aspire 3613 with Celeron M, runs cool.