Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
Once we upgrade the SEPM server how quickly do we need to upgrade the server agents? Our plan was to do them slowly during maintenance windows. Will 12.1.5 work with the 12.1.3 agents for a week or two?
--> Yes, you can plan with the help of Auto upgrade feature by scheduling specific time like non production hours. Auto upgrade can upgrade 10 clients in one go so if there are thousands of clients it may take week or more time to upgrade all the clients. Refer this article to know more info about it:
https://www-secure.symantec.com/connect/articles/sepm-121-auto-upgrade
Note: You must test the AutoUpgrade process before you attempt to upgrade a large number of clients in your production network. If you do not have a test network, you can create a test group. You can add a few non-critical clients to the test group and upgrade them by using AutoUpgrade. You can confirm the upgrade completed successfully by verifying the version number of the client software that appears in the About dialog box.
OR
If don't want to use auto upgrade feature can plan to push client packages during non production hours but it's a manual way to ugprade SEP clients.