Have you not heard what's happened with the 170/3s? I'm suggesting the same thing could happen again as DfT seem happy to let contracts to be negotiated between TOCs and ROSCOs without DfT involvement, even if the said units won't be surplus to requirements at the lease end date.
LM's units have lease expiry dates in September 2015 and Porterbrook have produced a brochure advertising their 172s to interested parties:
http://www.porterbrook.com/downloads/brochures/172 Brochure.pdf So operator x could come along and say "Can we lease your 172/2s and 172/3s in time for the December 2015 timetable change for a period of around 10 years", Porterbrook will say "Certainly. That'll cost you £x."
The 172s will be 5 years old by then - the same sort of age the SWT 170s were when they were replaced by cascaded 158s, or the newer CT 170s were when they were replaced by 158s on the Liverpool-Norwich route. It's not uncommon for newer stock to be replaced by older stock.