Well, I was feeling brave/reckless today, so decided to click the "Renew Now" button.
Much to my surprise everything worked as it should. As others have said, the initial offer is without landline PAYG which I have now and want to keep, but you get to add that back in right at the very end (£3/month).
No mention of migrating to EE or anything like that.
I've had text and email confirming my order is what I asked for.
In summary:
I had Fibre 100 + PAYG calls for £44.42/month
BT told me it was going up to £49.42/month
I renewed online and got it for £32.99/month
Let's hope nothing changes in the coming few days!
Great that you got a deal but just goes to show what a lottery the entire process is.