You could change the contract type from FFP to FP with EPA.  You would actually be changing the pricing arrangement, they contract type would remain FP. 

See FAR 16.103(c)  In the course of an acquisition program, a series of contracts, or a single long-term contract, changing circumstances may make a different contract type appropriate in later periods than that used at the outset. In particular, contracting officers should avoid protracted use of a cost-reimbursement or time-and-materials contract after experience provides a basis for firmer pricing.

Would you be changing only one line item?

Make sure that you speak with legal just in case a protest is threatened. 

Yes, you will need to modify the parent contract and the delivery order.

Always document, document, document.