To answer this, you'd need to compute the costs of the up-front engineering associated with trying to re-certify as a new type, and also the costs of the modifications. Additionally, there's always negotiation in the certification process: how will the manufacturer substantiate compliance, and if they can't, can they convince the regulator that the regulator's intent has been "met" within some acceptable level of risk? That can't be rigorously computed until the dialog with regulators takes place.

I don't know that Boeing has computed likely values of such costs, and that if they did, that they would allow them to be publicly known. Moreover, Boeing would probably focus on their worst-case numbers to justify to everyone why the only viable path is to minimally modify the design.