If your SA expired after October 1, 2016, depending on what program you acquired L&SA under, you could retain a perpetual license to use the LTSB. If you currently use the current branch, you must uninstall it, and then install the LTSB. There's no support to migrate or convert to the LTSB from the current branch.
If your SA expired before October 1, 2016, and you retained a perpetual license to Configuration Manager, then your only option for ongoing use is to install and use System Center 2012 R2 Configuration Manager and its available service packs. You're required to uninstall the current branch when your SA expires, and reinstall that earlier version of the product. There's no support to migrate to or downgrade from Configuration Manager current branch to prior versions of Configuration Manager.
If you use System Center Endpoint Protection, and your SA expires, you must uninstall it. System Center Endpoint Protection offers no L (License) rights, and no perpetual rights.