You most certainly can!
When we designed The Replicator ease of use, and maintenance were a big part of the design spec. All the injection molded parts have a snap on mechanism built into them. So what this means is you can unsnap the single extruder carriage and snap on the dual extruder carriage.
However there are a few other things that change when switching to a dual extruder system, that you will have to take into consideration. The addition of a second extruder requires a different bar mount to mount everything on, and of course you will have to purchase a second extruder to go along with it.
The other fairly important difference is the top filament tube constraint. There are two different ones for each style of extruder, and finally the last thing you will need is a Botstep to drive the second extruder.
Hope this helps ya out a bit if you have anymore questions just fire away =)
