Oooer, yep been there done that... I think I even mentioned it it my very first thread about being careful trying to undo that screwed hotend...

Well, I can say safely that the one that's broken off is well and truly boned, unless you know someone that can bore it out, then re-thread it and buy a new hot end with the right guage... ergh..

To change the block to the other side, you can unscrew the small black Hex nut that is on the main mounting bar and it should just slide out. Slide the broken one out the other side, swap them and re-tighten the small hex.
The downside is you are down to one extruder now.

An unofficial replacement dual head will set you back $150, one from Flashforge will set you back about $400

Having been where you are and having bought the replacement dual extruder myself, what I should have done is bought a re-furbished flashforge for $600!!