I currently have dsl service with ISP-A, using a modem provided by
ISP-A to connect to the Internet. I would like to try the service of
ISP-B and was wondering if I could do that by just swapping the dsl
modem provided by B for the dsl modem provided by A without canceling
the service provided by A. That would enable me to test B's service
to see how good it is. Is that possible, or do I have to cancel A's
service first before replacing A's modem with B's modem?

Thanks for any comments.
Work is the curse of the drinking class.