Thanks to all who contributed. I didn't get to the other links and helpful hints but just worked with what you provided and got it working so I will review the other options/ideas when time allows.

For lurkers, here is what worked (all irrevelant code removed):

preUpdateFieldValues<Dummy.Entities.Dummy2.VehicleStatusEntity>(VHStatusResultSet, currentEntity => currentEntity.MVehicleKey = "RMKL" + Convert.ToString(currentEntity.MIDKey));

Example of vfp it replaces:

preUpdateFieldValues("m.vehkey='RMKL'+ALLTRIM(STR(m.vehi2)))")

Note: IBusinessResult is the interface for a collection of entities

       public Dummy.Entities.Interfaces.IBusinessResult preUpdateFieldValues<TEntityType>(Dummy.Entities.Interfaces.IBusinessResult entityList, Action<TEntityType> replacementFunction)
        {
            foreach (TEntityType currentEntity in entityList.Entities)
                    replacementFunction(currentEntity);
            return entityList;
        }

Example of vfp code it replaces: Replaces the below section (tcFldChgMacro) in preUpdateFieldValues.prg:

LPARAMETERS tcFldChgMacro
 ….
SCAN
       Scatter memvar memo
       If !Empty(tcFldChgMacro)
             &tcFldChgMacro
       Endif
       Gather memvar memo
ENDSCAN
…