Ok, so I finally got a reply from Ofgem (via my FIT provider)...
"Thank you for your email.
Please note that within Ofgem’s role as administrators of the FIT scheme we must class any works that have not yet been completed as hypothetical situations and as such we are prohibited from passing any comment on this situation at this time. As such I can neither verify if this proposal will have any impact on this installations continued eligibility for the FIT scheme or offer support to any other proposal, until such a time as the works have been completed. Please note, as per section 8.16 of Feed-in Tariffs: Guidance for licensed electricity suppliers (version 13), below, Ofgem would also not expect a FIT licensee to offer a formal view on a hypothetical situation:
“8.16. We would not expect FIT licensees to provide a formal view to the FIT generator in respect of how proposed works will affect the participation of an accredited FIT installation in the scheme until full details of the works have been made available to the FIT licensee and/or the works have been fully completed. The effect of changes made to a FIT accredited installation should be assessed on a case-by-case basis.”
Regarding extensions, the relevant parts of the guidance would be 6.75. – 6.78, of which I would bring to your attention 6.77:
“6.77. If an accredited installation and an ineligible installation share the same meter then it is possible to pro-rate the meter readings taken from this meter.”
We would expect to be notified of any ineligible extensions for our internal records. The change in capacity would not be marked on the CFR however an installers letter or MCS confirming the additional capacity should be uploaded to the documents tab of the installation on the CFR.
In order for an installation to not be considered decommissioned, at least one part of the original generating equipment must remain part of the installation. For PV installations, the generating equipment is considered to be the panels and the inverter(s).
So it seems that upgrading the panels and getting pro-rated FIT payments is allowed, but only if the original inverter stays. I wonder what happens when the inverter goes bang, as they are known to do. Trigger's broom?