I'm trying to fly USA-JFK-TLV next year with 2 people in C. I found 2 seats at the low mileage tier from JFK-TLV-JFK, but there are 0 corresponding days with USA-JFK at the low level. I know as a PM I can refund the miles/taxes with no penalty up to 72 hours before the flight. Here is my question:

If I book the JFK-TLV-JFK legs at the low tier, and the USA-JFK flights open up at the low tier rate at a later date (but presumably the JFK-TLV portion then isn't available at the low rate), can I add on the USA-JFK leg to my existing reservation without additional miles and just pay any extra taxes?

Or, would they need to refund the JFK-TLV ticket and re-ticket the whole thing at the going available mileage redemption rate?