Like yourself, we hadn't changed ISP for years either, so the prospect seemed daunting. But very recently (last month) we took the plunge and made the switch from Coms to Zen.
Once we'd made our decision to migrate, and following an initial enquiry, (just to ensure there would be no problems on Zen's side) I submitted a ticket to Coms, to request a MAC code. The MAC was e-mailed to us within an hour or so.
The way it worked here, is that once I'd submitted the order with Zen, they contacted Coms to give them a completion date. A Coms Senior Analyst then e-mailed us to say they'd received notification of our order being placed with Zen, and confirmed the completion date. (I would imagine that Xilo and Coms will communicate in much the same way.)
To be fair, Coms were actually very helpful throughout the period between the order being submitted (5th Sept) and our migration on 22nd Sept. It all went very smoothly, with no problems, so hopefully, you will enjoy a similar experience.
Wishing you all the best.