Does the isp switch off your bb as soon as they give you a mac number?

Im guessing not, but I wanna make sure Im able to order a new product online without getting a 56k modem to use.

Also, does the migration process involve any downtime while youre switched from one to the other?

You should read on adslguide, some people report it taking 6 to 8 weeks with 2+ weeks downtime

It all seems to depend on your ISP, when you cancel in relation to your billing period and who you are moving to :/