The moral of the story as far as I know is that in order to rent out a room under the rent a room scheme, you need to be living there, OR be deemed to be living there.
If you are being forced to move abroad or to another part of the country for employment reasons , you can live somewhere else and keep your apartment as your PPR, therefore still being able to avail of the rent a room scheme (i think! Can anyone advise?). I'm not sure this is applicable in your case however.
It looks as though you really do want to move apartments, so if you are determined to move it looks as though you'll have to give up your PPR status on the current apartment. This has pretty serious financial implications as Clubman so professionally pointed out in that link.
My advice - try your hardest to keep the current apartment as your PPR even if you dont plan living there, rent out one room and hope for the best!
Remember tax avoidance is diffreent to tax evasion!