"04"MC, "05"EC Whats the diff ??
 

I have a 2005 gasser EC 250 thats well on its way to getting a 300cc upgrade. My "05" has all of the LTR goodies except for the exhaust spigot. I also had the Squish-band sdjusted at RBD.



Is a 2004 Gasser completely interchangable with a 2005 Gasser ?

Think its Enduro and Motocross. Jetting is different, and wiring harness should also be different (note; more wires to EC; lights, blinkers etc.)
Suspension should also be setup some different, or other shocks.

Other than that, the same.

If you are doing a full motor swap it won't work the bottom motor mount is different from 04 to 05

I actually have an 05 frame setup for an 04 motor in the basement

It is my understanding that they made a small change to the frame geometry going from 04 to 05.
Pretty sure the MC's have four speed transmissions.
My 04 is a DE (Dealer Edition) and came with the mx ignition. Around this time they gave the dealers some options on how they wanted the bikes so they were not all exactly the same. Some had lights, some didn't.
I would think that swapping top ends would not be a problem.

The main thing to remember with GasGas is that the model, can mean something but more typically means little.