How to replace a large chunk of geometry

I have a level where I want to replace a section of it during gameplay. The gimmick is that a ruined stone building is in disrepair but through some ancient magic, once it’s guardians are destroyed, it’s restored to a highly polished sci-fi look.

What is the best way to accomplish this? I need to make sure that the navmesh and lightmap appear seamless. I’ve considered breaking it into three scenes (the static geometry, the ruined one, and the polished one) and adaptively loading them. That should make the light maps work automatically… I think? I’m not sure how navmeshes would work in that situation though. I should be able to stitch them together with off-mesh links?

Alternatives, thoughts?

I have only tried addaptive scenes from the lightmap aspect. It works. Also now in 2019 light probes work correctly. Though fro them to load in the host scene you need to call a bit of code, which I dont recall exaclty now but its easy to google. After that it seem to work fine. Dont know about navmesh though.

Also depending on the scene its hard finding good transitions between different scenes., light and shadows tend to be overlap into multiple scenes. edit: more so if you have different ambient setting, like you try to bake outdoor in one scene and indoor in another.