What’s the current contractual rent, and does the landlord intend to increase it?
If there is currently a shortfall due to the 2x shared room rate and this arrangement is purely intended to reduce the shortfall, it might be hard to argue that there is the element of impropriety necessary for a “contrived” decision. There have been previous discussions on the fine line between maximising income and taking advantage of the scheme. I would say it depends on the extent to which the landlord is taking the mick.
Yes the tenants would have to agree to terminate the existing tenancy, this is why I suggest you discuss it with them first. I probably wouldn’t tell them its definitely all or nothing… but there is a risk of reg 9 biting depending on various factors.