Sure the fee level must be included as an alternative of the fee hash. I imagine apart from that nothing within the invoices must change.
Invoices have numerous fields with numerous semantics. One in every of them is the sphere for the fee hash. So as to keep backwards suitable I assume there can be a brand new area for fee factors (public keys) as an alternative of simply placing a degree inside the sphere for the hash. On this manner somebody decoding the bill may already see if the bill was for ptlcs as an alternative of the hopefully quickly to be legacy htlcs.
AFAIK till now noone has proposed a concrete bill format but as we nonetheless have not agreed upon a selected manner on tips on how to implement ptlcs in lightning