Hi Clint,
1. If you have "Legacy Agent Communication" enabled, then all directed clients where SMA 7.1.x isn't yet upgraded to latest 7.5.x version, should communicate with your SMP 7.5 Server and receive policies/tasks.
2. I don't have list of all things to enable, but if you are using SMP + Solution(s) and there are Site Servers (PS, TS), then:
-- 1st: Enable SMA upgrade rollout policies to upgrade it on clients and Site Server(s) as well.
When Package Server will be upgraded, make sure that "NS.Package Refresh" scheduled task is executed on SMP Server to have all latest available package codebases for further delivering them to managed endpoints
-- 2nd: On Site Server Management page, enable required Site Server(s) upgrade rollout policies.
-- 3rd: Find other Solution(s) plug-in upgrade rollout policies and enable them. As example, Inventory Solution plug-ins upgrade rollout policies, etc.
Thanks,
IP.