Unfortunately having different runtime versions in a single project is not possible [*].
I fear if you want to use spine-unity runtime version 3.8, you have to reexport your assets from Spine 3.8.
I assume you already know, but just in case:
You can use batch-export to re-export all your skeletons quickly via the command line interface:
Export - Spine User Guide: Command line
[*] It is theoretically possible, but would involve a lot of programming effort on your side. I once posted the necessary steps at a blog post for the sake of completeness here:
Two Spine runtimes in the same Unity project?
In short: please do not attempt to mix runtime versions.
