Sounds ok to me, if a little inconvenient both for you and Demon.
As kitz has already said, it's possible for an ISP to place a simultaneous migrate and regrade order, which means they can change your speed during the migration. It looks like Demon haven't done that. Doesn't surprise me really, because they're usually quite slow on the uptake of new stuff.
It sounds reasonable that if they've now placed what's known as a Modify order into eCo (the BT systems that ISPs can talk to), then your order should be done tonight, and your router will start connecting at a higher speed. You should see a faster upload speed immediately.
However, don't be surprised if you don't start seeing the new faster download speeds until at least 3 days, sometimes up to 10 days, after you are Maxed. This I think would be applicable even if you'd been done on the simultaneous migrate/regrade path.
So them telling you to do a speedtest on Friday is a little optimistic, IMO. Don't be disheartened if you don't see faster speeds until well into next week - it might even be the weekend after.
Of course it might happen a lot sooner, fingers crossed for you