New Server, old agents

Asking for another school, before I give him the wrong information. I know how to update agents through System Updates, but if his OLD 11.1 server crashed, then re-registered them with the NEW 2017 server, will it show up as a System Update to deploy to the old workstations?

I know at one point I deleted a base update, so it could deploy the SP, so had to re-import the original update, but wasn't sure how to proceed with a newly built server.

To simplify:
Old 11.1 crashed
New 2017 built and machines registered
How to update 11.1 agents on existing machines?