As a former BLT CM, I might be able to clear some of this up. What you saw on the tour is indeed creative costuming, which handles the costumes for all the parades and shows on WDW property. Not sure if they do any work for shows at other Disney locations but I would assume they might from time to time. As far as I know, the place was pretty busy during the week with probably 10-20 people working there, and that's just in the showroom visible to guests. There are several offices located upstairs and behind the showroom which I'm sure had several people working there as well. I would think those people are still there, and will need to be relocated, though I doubt a brand new facility would necessarily need to be built.
As for the rest of that building, I don't think anything else would need to be replaced. The prop workshop was essentially just a satellite location for central shops, which is a far bigger facility located behind the MK. Any work being done there could just as easily be moved to the bigger location. The rest of the building was for storage; many of the Christmas lights were stored between costuming and the workshop, and all the rooms on the right side of the tram were used to store tires for LMA.
Some people have mentioned wardrobe, and this is the place where CMs pick up and drop off their costumes. It is located in the Cast Services building directly behind Indy. If the rumors are true about SWL expanding into the parking lot, that building will need to be replaced somewhere. I would guess they will put it wherever they plan on relocating CM parking. If it were me, I would build a brand new lot and Cast Services building across World Drive just north of where they are planning on building the fireworks launch site. You could even include space for Creative Costuming there as well.
