Well we're looking to get the HPP and RHI so need to completely replace our system.
You can install a wood-burner and retain the fossil-fuel appliance for emergencies, whilst still qualifying for the RHI; told that on the RHI biomass course, so it's probably right.
You didn't tell us whether you own the local forest.
You might want to go and see one of these working. The rate at which they gobble wood when going full blast is quite impressive and would put any ideas about casually collecting firewood into a realistic perspective.