Since the processor is different for 261x and 262x XM routers, it can't be done, as mother board needs to be changed. Also 262xXM has two 10/100 fast ethernet ports while 261xXM has one.
You can visit following link to compare the modules which is supported in both the series of routers http://www.cisco.com/univercd/cc/td/doc/product/access/acs_mod/cis2600/hw_inst/2600hig/2600ch1.htm
Is there any special reason that you want to upgrade like that?
