I think that's just how it is I'm afraid. Mobile phone contracts are no different; when you switch on those, you are locked into another 1 year (or what ever; certainly was the case when I was with EE).
What is shady here is that you moving from 80/20 to 40/20 means Plusnet do not need to submit any new order for your connection to Openreach to adjust your connection, thus, the Openreach ordering system will not lock Plusnet into another 12 month minimum contract for the FTTC product to be provisioned on your line. Thus, Plusnet are simply enforcing another 18 month contract on you for the sake it; there is no underlying reason why they need
to do this.
So yeah, your call. Jump ship now or stay, the choice is yours.