It's the FNOs. Almost all of them.
Since downgrades are normally rare, ISPs price them into everyone's monthly rental. So you help pay for other people's downgrades.
But with the forced upgrades the FNOs are doing, ISPs have to eat it or piss off the customers.
Upgrades and downgrades should be free from the FNOs side. It's not like humans on their side have to do anything. It's all automated.
You have experience in setting ISP and FNO pricing? And understand how to upgrade / downgrade customers?