The only provision for re-opening the implemented quotation would be if a Project Managers assumption, on which the quotation was based, has proven to be incorrect.
Even if the quotation could be reopened, the share of the saving would not fall under Defined Cost so would not be included in the assessment.
I would therefore suggest that an agreement is recorded with the Employer under clause 12.3 that a proportion of the saving is one of the other amounts to be paid to the Contractor. This would then enable payment under 50.2 as you suggest.
Even if the quotation could be reopened, the share of the saving would not fall under Defined Cost so would not be included in the assessment.
I would therefore suggest that an agreement is recorded with the Employer under clause 12.3 that a proportion of the saving is one of the other amounts to be paid to the Contractor. This would then enable payment under 50.2 as you suggest.