Think the Chiltern units are to increase capacity (now there's a novel idea!), so don't think those will free anything up. The GOBLIN line is an obvious infill electrification for which momentum appears to be growing, so maybe the Class 172s will themselves become available before they are barely built. (I'm surprised the GOBLIN line hasn't been electirifed as part of the flood of pre-Olympics funding in the area.)
So the 172's could potentially be up for grabs?