Replacing in-situ felt isn't easy. You basically have to take the entire
roof covering off, complete with battens. At least that is how they did it
on my loft conversion before relaying breathable membrane and reclaimed
slates.

P.S. Cost wise I would guess about 2000, assuming the existing roof covering
hasn't got too many broken tiles and the pattern is still available for
spares.

With new tiles/slates, add another grand or two. More still for Welsh slate,
rather than artificial or concrete.

Christian.