One quite famous example of how this re-fit method can work properly is the Falmouth Quay Punt 'Curlew', which was originally built more than 100 years ago.
She is now on display in Falmouth, Cornwall at the Maritime Museum; her previous owners gave her a new lease on life approx 35 years ago in New Zealand where they sheathed the original timber hull with a new hull built with 3 layers of strips of kauri timber bonded on with epoxy.
OK, not 'fibreglass' as such, but the same concept.
Some info about her -
Falmouth Quay punt "Curlew" – BC09 | National Maritime Museum Cornwall https://nmmc.co.uk/object/boats/falmouth-quay-punt-curlew/
And an excellent article about her by the Carrs -
We Uncluttered Our Cutter https://www.cruisingworld.com/how/we-uncluttered-our-cutter
I saw her sailing in Antigua Race Week in 1987 - this was before the Antigua Classics Regatta was thought about, so Curlew was racing against the hot IOR and other fancy racers of the day. And she hammered many of them most convincingly - she won her class (where she was the only gaffer / long keeler). And if I may quote from the article, 'she also won second Best Performance Overall, outshined only by a souped-up, flat-out, Admirals Cup Two-Tonner'.
Not bad going for a 90 year old!