The whole issue of how the FITs scheme handles incremental upgrades over time is completely open. The report recognises that the situation can exist but only says this:
Installations over multiple years
3.138 There are likely to be instances where generators increase the size of their installation over time. For example, they may install one wind turbine in year 1 of
FITs and install another wind turbine in year 2 on the same site, increasing the total capacity incrementally over a number of years. This creates issues with how
the total capacity of the site is treated for the purposes of tariff banding and how degression is applied to the newly added generation on the site.
and then poses 3 questions:
Q61. What do you think is the best way of defining an installation for the purposes of FITs?
Q62. Once an installation is defined, do you think further checks are required to verify this? If so, what would these checks be?
Q63. How could we deal with installations at a single site installed in different years?
So you can see that the report has no suggestions as to how to address this situation.
I suggest you respond to the consultation indicating that you are a system owner in exactly this situation and what you would like to see happen from a practical point of view.
Personally I think an equitable solution would be for the generation from the panels to be 'pro rated' across their different years. So, if you started with an existing PV system of 2.1kW, which migrated from ROCs, and then added 0.4kW in the first year of FITs your total generation would be paid at:
84% (=2.1/2.5) at 9p and 16% (=0.4/2.5) at 36.5p
In other words if you generated 1000 kWh you would be paid 840 units at 9p and 160 units at 36.5p for a total of £134 so you would be paid an 'effective rate' of 13.4p per kWh.
I can see that from an admin point of view that this would be less than straight forwards, but not impossible, and that policing might be difficult too (maybe you would have to submit a copy of the installers invoice when applying to register a system upgrade).
Anything that left you in a worse financial situation than this could lead to the ridiculous situation where you decommission a working 2.1kW system and recommision it as a 2.5kW system and then get 36.5p on all of the generation. This, I am sure, would lead the FITs scheme into dis-repute.