Hello,
im vondering if i could move generated certificates to another older server where letsencrypt is not working anymore... to old system...
and yes i know.... but just for couple weeks to migrate to new server...
Sure you can, it's just a certificate like anything else. Locate the files (key/cert) generated by Let's Encrypt and paste them into the certificate boxes on your other server. You may or may not need to include intermediates ("CA certificate").