Let's assume, given how long it took 4400/4402 to return that there is a lot more going on than ramp replacement, which surely could have been done at the Barns anyway.
Maybe the process will be that La P will do disassembly then ship 4401 in "kit form", possibly along with La P manufactured parts, to be reassembled on TB's line. This does not tie up either of TB's lines or other floor space to do the disassembly but may take advantage of floor space available (for whatever reason in La P).
Alternatively, the attractive factor may be the availability of skilled workers in La P pending the ramp up to the new Azur order (rather than the floor space)? [EDIT: there could be pressure on BBD to use La P for Quebec City trams if BBD gets that work, so it works on a familiarisation basis also]
If that's the case I wonder if it's more economical just to renumber one of the new cars as 4401 and eventually have the usable original components recycled into the production line as another car