Hi and welcome to the forum!

Is the system registered to any repository management like SMT/RMT or SUSE Manager? During the upgrade the default way of the installer is to connect to SCC and update the repositories according to your subscriptions. It's not really optimal but one can usually work around that. Can you show the output of (please mask all sensitive data):

SUSEConnect -s

zypper lr -Ed
Depending on your actual setup the SUSE Distribution Migration System might be worth looking at. But let's see your repo setup first.