reading migration guide.

the best approach is to load the old data somehow, then write it with the newer version. For example, by juggling classloaders the data could be loaded with v2, then written with v5.

I fail to see how this is the best or even a good approach. Could you please elaborate?

As far as I see it's best practice for java libs in general to simply move to a new package for major / breaking updates like this eg. com.esotericsoftware.kryo.v5, so everybody is happy with smooth migrations.