Guess we will have to wait for an answer from Zen 
But if line rental and FTTC could be treated as two separate items on one's Zen account then any potential future change to a FTTC and line rental package would involve the 30 days minimum term on line rental being changed to a 12 months minimum term and that would perhaps have to be a new contract agreed by both parties?
If Zen decide to change the underlying means of delivering your services, they wouldn't be able to change the contract terms unilaterally, though there are certain circumstances where the terms of your contract might allow new terms to be imposed when accompanied by a right of termination if the changes are to your detriment.
The contract between you and Zen may well not prohibit Zen from changing you to MPF where that is technically feasible (i.e. lines where there is no broadband or where there is Zen broadband). If, in the future, Zen launched an MPF phone service and chose to migrate you from WLR3 to MPF, you would be bound to your contractual minimum term and Zen would take the risk that their choice to migrate you could expose them to BT Openreach early termination fees for MPF.
My post was really just a 'heads up' that Zen may go down the MPF route in exchanges where they have presence, and it's best to bear that in mind if you wish to purchase phone and broadband separately from Zen with the intention of staying on WLR3.
As you say - over to Zen for the definitive answer, if there is one bearing in mind I'm hypothesising about potential future services.