I think this is the critical issue that was debated in the recent Tribunal of Commissioners (see summary in A13/2006 and other discussions on this site). The C’s pointed out that there is no need to target one person for recovery – provided your decision letters meet the requirements.
Secondly, I personally see no reason why you cannot now change the recovery target and I think you can do so back to 2001 at least (based on the new case law). You must of course issue a new decision and give the right of appeal if there is one. It is my view that LA’s are now entitled to review all OPs created over the last five years (where payment was paid direct) and switch recovery to the landlord.
Naturally, this will not make you very popular as the LL will not even be able to appeal to TAS on the main points (maybe perhaps on the issues surrounding your right to supercede / revise) and would have to apply for judicial review. On the other hand, it seems to me that the Commissoners are taking a different line. Recovery of public funds is important and is a priority is one of their key messages as I read the decision.