Under FiT guidelines, you get paid for being a generator, and paid for whatever you export.
You won’t be able to receive FiT export payments whilst also being on the Outgoing Octopus tariff. However, you will still be able to receive your generation payments. When you sign up to the Outgoing Octopus, you will be required to opt out of your deemed export payments from your existing FiT supplier (but there’s no requirement to switch your FiT contract to Octopus.)
For new installations (that is, after the FiT closure date of 1/4/2019) we’ll still require a copy of your Microgeneration Certificate Scheme (MSC) certification in order to get you onto the tariff.