To renew Felt on shed roof

Only if the boards under the felt are wet or rotten will it need reboarding

Yes one change in any form of flat roof we advise that you change any form of timbers underneath which might be wet or damp otherwise all you’re doing is putting a plaster over you infected wound it will still fester and causing more damage in the long run

Yes I would change them, if you seal the shed roof it will trap the moisture in and give you problems further down the line, many thanks Andy Smith Phoenix roofing & Landscapes Services

Yes changing them would be better as the roof would have more strength and over time the roof will sag and could fall in and hurt someone.