Would changing ISP actually involve a new installation? If it's just a switch of retailer on the same kind of line, I wouldn't think it would take much? If it's going from, say, ADSL to Cable or something, I assume that's a more difficult process.
nb I am not IT savvy
Different companies run their own cable from the street. So depending on the configuration of the building, and what's already there, it can be quite a bit of work, and require drilling holes in walls and such.