Old keys are missing dag-cbor data field

I have been using go-ipfs 0.6 with generated keys for years.
Now I have migrated to latest kubo.
I can still use my old keys but cannot use them to publish to ipns.
ipfs name publish -key=“mykey” …
returns following error:
Error: record is malformed; record is missing the dag-cbor data field

So I assume that old key is missing something that new keys have.
Because I would like to use my old key (to keep my same ipns address)
my idea is that it should be possible to convert old key to plain text format, then add this missing data field and then convert key back to binary format.

How to do this? Please help.
Will keys change again in the future so that we have to throw away old ipns addresses?

I suspect that your node is trying to republish an existing IPNS record v1 from the data store. Since IPNS v1 records are now deprecated, your best bet is to to try to force Kubo to create a new record.

The easiest way to do that would be to run the ipfs name publish command with a --lifetime or new path.