Dcal is basically v1 of chem with a large number of features added. Chem 2 is the newer version of chem. I would suggest that you use both, or at least look at both and decide which one you prefer. The fuel scaling in chem 2 is very nice and extremely easy, just a couple of clicks and your fuel is scaled for whatever injectors you're running. Dcal on the other hand, will do flashing for smecs (assuming you have a flashable smec). Both will allow you to edit the tables and constants in a calibration, provided you have the proper .tbl or .calx for said calibration. There are some other minor differences, but both programs are useful, and neither should be ignored.
To modify your lm, you basically need to pull the chip with the cal on it off, solder in a socket, and pop in a new chip. As to which chip to remove, it depends on the year of the LM. I personally use 27sf512 chips to burn the calibration on, depending on which year, you may need to double/quadruple your calibration. There's some info at the link below
Electronics - Automotive