The size difference is too large. A new refit rule in C# means that you can only refit to a design that is less than 20% smaller or larger than the design you are refitting to.
I have my original GSV survey vessel orbiting Earth , fully fuelled and overhauled. I have retooled a Shipyard to construct an updated GSV vessel that includes a Jump Drive. The Shipyard had a capacity of 5000 tons and the updated vessel is 4001 tons , up from the original 2358 tons.
However when I try to use the refit button there is no option to refit from the original to the revised vessel - the only solution at present is to construct a new vessel from scratch - any offers as to why this should be.