You know what happens when we assume. 
It's good that you posted before attempting a conversion.
Wiring the motors in series cost nothing, which makes it very cost effective, and can be easily reversed.
Wiring motors in series:
The motors have two wires attached to them. They could be any two colors, but for purposes of this discussion, we will assume that each motor has one blue wire and one yellow wire, OK?
-take off the shell
-Follow the blue wire from one motor back to the board and snip that wire at the board.
-follow the yellow wire from the other motor back to the board and snip it off at the board.
-now you have one motor with a long unattached blue wire, one motor with a long unattached yellow wire and two wire stubs on the board.
-take the long unattached blue wire from one motor and connect it to the long unattached yellow wire from the other motor. You can connect them with wire nuts, or solder and heat shrink.
-put some tape or heat shrink on the stubs of wire on the boards if they can touch anything.
-test drive the loco before putting the shell back on.
That's all there is to it. Your loco will run more slowly and be less inclined to slow down in spots where the track voltage is low. The lights in the loco and the cars will be brighter.
There are other ways to do it, some that are more easily reversible, but this is the quickest/easiest way.
