I Just read that an offline update from SUSE Manager clients to SLES 15 (from SLES 12 SP3 >) is not supported for Salt Clients. Can anyone confirm this? If it is true are there plans to remove this restriction at some point and does his mean all salt clients will currently require rebuilds?
I remember them stating in a webex back when 15 was first announced, that if you built a 12 sp4 you could go to 15 but not any version prior. Those would require a rebuild, though that could have changed since. On the other hand 12 is still fully supported, with SP 5 current and I believe there are plans for 2 or 3 more SP’s for 12. We have applications that will continue to run on 12 because not all the packages from 12 are available on 15 for one reason or another.
Useing SUSE manager to migrate is possible from 12SP4 upwards only. And even then its not straightforward to get working, it took me a couple of calls to support. I ended up doing this using cobbler and PXE booting the systems to pick up the upgrade.
I tried the inplace upgrade from sles12 salt-client to 15 in january 2020. During this time the documentaion did not mention salt, so I thought the upgrade via salt is supported 
the upgrade started, but did not finished succesfull.
the os-upgrade was done, but the reregistering did not cleanup the channels.
During the upgrade the communication between suse manager and the client got lost.
I opened a call, but they did not understand.
In the meantime (since SUMA 4.0.5) api-commands are available, but the documentation did not show any examples.