Cooling is particularly important of course. Often you will need to upgrade the heatsink and/or fan if an upgrade is available. Just check the parts lists for the model in question and you may find an upgrade unit that is intended for the faster CPU.
In addition to that, you generally want to get rid of the adhesive heat transfer pad they often use as an interface between the cpu and heatsink. Just get some copper and polish it, polish the heatsink as well, then use your favorite compound for mounting the heatsink to the CPU, just as in a desktop.
It is obviously more difficult to improve cooling in a notebook than a desktop, but it can be done with straightforward means.
This is important not only in upgrading processors, but any time one wants a good performing laptop....perhaps for gaming etc. In addition, it is vital if one is to overclock. Yes, you can overclock laptops.
upgrading a pentium4 system to a pentium-m or pentium-m to core processor you can usually forget, as new processor generations require different motherboard chipsets, vrm specifications or microcode support.
Yup...gotta stay within the boundaries. Funny though....the boundaries are often a little farther out than the manuals will tell you. In the end, it's going to come down to what the BIOS will recognize...often, that is the limiting factor...but not always.
I was talking to a Dell tech one time about a CPU upgrade I made and he said "you can't do that!" I said "of course I can, and I did" He asked "how?"
I told him I used a hammer.