Then I decided to switch to the 100/50 plan, so I called them and asked. No problem, they said, but you'll have to wait until your billing anniversary of 22 Feb.
Come 22 Feb - rebooted everything and tried the mighty speedtest, to make sure the change had been implemented.
The result? Well, there's been some change, my connection is asymmetrical, but that's about all I can say

They've successfully adjusted HALF my plan...
Very weird.