If you port away the number that your DSL service is attached to, the DSL service will be canceled.

So the best procedure is to order "dry loop" DSL service in advance. This is DSL service that is not bundled with a phone number.

If possible, I'd have it setup on an entirely new billing account, just to be safe. Or at least try to make it clear that you will port the number out and do not want to close the dry loop DSL.

Once that is setup, have the DSL service removed from the number you intend to port.

Then you can freely port the number without any DSL downtime... theoretically... losing providers have a nasty habit of mucking things up.

Another option is to move to a fiber or cable internet provider, if available. Once installed, you can port your number and let the DSL go away.

Good luck.