I haven't seen this scenario before, but if your order says Infinity then you will either get it or the order will be cancelled again. They won't drop you to ADSL2+ just be default.
The faster broadband with fibre will be fine if they do cancel, but AIUI that would be a further order which you would arrange by speaking to them. It's a complete PITA.
I expect others will be along later who may know more. But good luck. You won't end up on ADSL2+ and stuck in contract. As long as it is Infinity showing on your order.
I also don't want to be without Internet for weeks because BT cancel my order just to keep their stats up.
If I end up with a phone line contract for 12 months but have to go to another supplier for internet that's also a pain.
This all feels very misleading to be honest. Tell me I can have a product and then cancel my order despite the fact it would work here just because my slightly slower speed means they can't claim their average speed is as high as they want to compete with competitors.
At least with other service providers they provided what they promised.
I'm also unhappy that BT don't keep me informed. I have to chase them. They're meant to be a premium provider and they're one of the more expensive options. So far service has been poor.