I use the following footswitch with my DC-5. Depending on what version you have, you might have to change the LED wiring in the switch.
These are very sturdy, steel housing, and cheap.
www.tubesandmore.com
FOOTSWITCH BOX REPLACEMENT FOR MARSHALL TWO BUTTON WITH LED
P-H471-LED
Two button footswitch replacement for MarshallTM with LED.
$21.95
The Y splitter should work as well, but I hate having too much crap on the floor.
As for the changes, there were also power supply changes, and footswitch wiring changes. On the older ones, the EQ was only foot switchable from the back.
There may have been other refinements as well.
Each circuit board has a model # & a revision # printed on it. The best way to tell is to take note of how the EQ is switched, what type of switches are on the front, and looking at the circuit board.
MAKE SURE THE CAPACITORS ARE DISCHARGED BEFORE REACHING IN YOUR AMP CHASSIS.