A lot of this is answered in the Plusnet
Terms and Conditions which you signed up to when you took the service, and it gives Plusnet broad authority to do this.
"16. Our right to suspend your service or end our agreement
16.1. We can suspend or stop providing our services and/or end our agreement where:
16.1.1. our ability to continue to provide the service to you is materially and adversely affected because: (i) any of our telecommunications carrier(s) or supplier(s) ceases to provide services to us; or (ii) any authorisation required by us ends or is changed;
[...]
16.1.10. we are no longer providing the service to customers.
[...]
16.4 We can terminate our services and/or end our agreement at any time by giving you 28 days' notice."
Under this part, Plusnet could have said "we are terminating the service because Openreach are no longer providing WLR". However, instead, they've chosen to allow people to migrate seamlessly to EE, or to drop the voice part of the service.
The contract allows them wide scope to make such changes:
"18. Changing these terms, prices or our services
18.1. We may change the price, or other charges, the service or the terms of our agreement:
18.1.1. if we materially change the service, or introduce new services, features or benefits (for example if we increase the maximum upload speed for your broadband service);
[...]
18.1.4. to reflect changes in technology (for example if we develop new systems which provide you with a better service);
[...]
18.1.7. due to any other change in circumstances in the future, that we can't predict, which means a change is necessary."
The question is, does this allow you to exit the contract penalty-free? Well, it depends. There is this:
"18.6.3 We’ve changed the service in a way that is not exclusively to your benefit
[...]
If you give us notice to terminate this agreement for any of the reasons set out in this clause (excluding 18.6.6) and 18.8 then the agreement will terminate on the day before the proposed change comes into effect. Where that is not possible, the agreement will terminate as soon as reasonably possible, and the change will not apply to you."
It's then down to a discussion around whether an imposed migration to EE is exclusively to your benefit ("EE is the better provider" etc). You could argue that the hassle of having to swap routers and reconnect your phone line and/or rewire extensions is not to your benefit, and I think that would be fairly strong. Even one minor inconvenience means the whole bundle is not *exclusively* to your benefit.
Therefore, with enough complaining, maybe you could get an early contract exit - if that is indeed what you're looking for.
However, Plusnet might instead consider it as "transferring" the agreement, especially since the financial terms are being carried forward. In which case:
"19.1.[...] We can transfer our agreement to another company provided this does not adversely affect your rights under the agreement."
Then it's a question of whether being moved to EE "adversely" affects your rights - and in this case, it's not necessary that such a transfer must be *exclusively* to your benefit. You'd have to argue a significant adverse impact overall.