Lynn sorry I'm not to sure what you mean. Can you explain a bit more?
do you mean that you want to hold on to your current home and rent it, and buy an affordable one for you to live in?
Or jointly sell your current home and apply for an affordable home in your own name?
The AH scheme is mainly for FTBs but others can apply if they have special circumstances (e.g. divorced so buying on their own). I'm not sure if your circumstances would fit their criteria (but there is no harm in asking).
Even if you sold current home (and went into negative equity) and were approved for the AH scheme, you can only have an 80% LTV ratio- so you could not get a larger mortgage and pay back your old mortgage deficit and buy new AH home. You would probably have to get a personal loan (??) to cover the deficit.
BTW- I'm not entirely sure, so am open to correction!