Originally posted by CapNKirkland:

is there any way of copying all the buildings and bases i've made in my current seed and paste them into an updated seed?

or is that not possible?

The PlanBuild[valheim.thunderstore.io] mod can copy and paste buildings, including saving to file so you could take it to a different world. I think, but I'm not certain, that there's a way to copy terrain as well, so that your building is stable when placed.

I've never used it to copy terrain or across worlds though, I'm pretty sure it can do what you want but I don't think I can help you beyond suggesting it.