With my experience, I have found the about 90% of laptops you can actually upgrade the processor. The select few that you cannot upgrade include smaller laptop such as 10-12" screen and Mac Laptops. There are small companies that would do this, but there is not guarantee that you will have a working laptop in the end and the cost is way over the price of buying one of the higher end macbook pros brand new.
As for Lion, I am running it on two of my computers, my macbook and Macbook Pro. the macbook the hard drive died before installing the OS, so i figured out away to do a clean install rather than install Leopard on other mac (lost original disc and retail does not work on this model), move the hard drive back to the macbook, then upgrade to Snow Leopard, then update to 10.6.6 or newer, then upgrade to lion and do it's update. Well the MacBook runs great, but the MacBook Pro which was upgraded From Snow Leopard is terrible, which is suppose to be much faster but is not. There is problems with safari and other programs. Also, they took more programs and features out then they added in.
So either know what you are getting into and losing and buy a new Mac or stay with Snow Leopard.