Apologies if somehow this is already happening and I missed it. It would be smoothest and fastest to update to newer LTS of Stackage if stack knew to check the version differences and just copy compiled binaries from previous LTS for unchanged items and only compile updated ones. (Maybe to save space could even just link, although that would make each LTS directory less independent, would cause breakage if older LTS directories were purged, although there could be a function that would copy over linked items prior to purging old LTS collections…)

I understand if most things are changing too often for this to work smoothly enough, like dependencies of dependencies etc. — I suppose it'd have to check and recompile all items that had any of their dependencies change.

If this isn't practical, I understand. But maybe it's feasible…