Andrew,

Somebody more familiar with the particulars of this model ay be able to give you more pertinent information, but I find it peculiar that Roco would list a decoder for the Maerklin Motorola AC digital system as a replacement part for a DC locomotive. Either Roco has been a little sloppy in compiling this parts list, or what you have is actually an AC locomotive that has been converted to DC operation (this only requires removing the central pickup shoe, replacing the AC wheelsets with DC ones, and re-soldering a couple of wires, all relatively easy to do - and often done.)

If the decoder really is a Motorola decoder, it will not be of any use to you when you go digitial, unless you go to Maerklin Motorola AC digital instead of DCC, the usual choice for analog DC conversions. On the other hand, if it was fitted after initial purchase, it may be a DCC decoder, or even a multi-protocol decoder that supports both MM and DCC - both of which would work for you.

Do you have a model number for the locomotive? Was it new when you purchased it?

Tom