Can I use the certificates from the old server during QlikSense Server Migration?

You can import the QlikSense certificates exported from the old server as long as the new same has the exact same FQDN (fully qualified domain name).

QlikSense uses certificates to build trusted connection. The server's the FQDN is binding with the certificates.

If the new server has a different FQDN, the old certificates are not valid any more and you have to run the bootstrap command to generate the certificates against the new server name. (How to recreate or just delete certificates in Qlik Sense)

Accordingly, you are required to re-enter the login confidential for data connections and UDC in QMC if they are using old hostsname or encrypted on the hash from certificates.