Hi @Tatyhead and welcome to this customer forum. It might be worth double checking but I wouldn’t think that a diverter would alter the main installation and Ofgem do allow additions now as well so it should be ok (in my view)

FIT doesn't put any limitations on how you use the electricity you generate. You can fit a diverter, battery, solar EV charger or any other device that makes use of your electricity.

I am no expert. If you install a diverter will that mean your exports will be less, and as you have a smart meter, so your FIT payments will fall a bit as you will export less electricity?