Accurate data for RUPs for replacements before FTA has been updated

FTA usage can't be updated until the start of the calendar year.
Current process is for RUP for replacement in future to pick up last year of the current agreement and move that forward into all years.
Problem: if the last year of current agreement is part of non-use (which often changes annually) then the usage data for the "future" RUP is incorrect and there is no way of editing it.
Solution: ability to edit "future usage" years until the new agreement is updated in FTA and then the new usage data pulls over --> and an amendment is triggered if that data is different than what was entered (some sort of "compare and warn of changes" thing

Tangentially related to #745

No files or tests are named. Start by tracing how future replacement RUP usage is populated from the current agreement and how the FTA update reaches that data; done means future usage years can be edited, later FTA changes are compared, and differing data triggers an amendment warning.

Written by the indexing model from the issue text.