Hi Morris,
You don't need to uninstall the SEPM. If you want to migrate your SEP clients from one SEPM to another SEPM, you just need to import the communication XML file from correct SEPM on your target clients.
Steps:
1> Export communication xml file (Sylink.xml) from correct SEPM. Copy it to desired SEP clients.
Exporting and importing the client-server communications file (Sylink.xml) manually
2 Importing Sylink.xml into target SEP clients.
Cannot manually copy Sylink.xml into a Symantec Endpoint Protection client. Access is Denied.
And YES, you can manage different SEP 14.3 versions clients under the same SEPM. However, you should consider compatibility of different versions of SEP and SEPM.
For example, if the SEPM is v14.3 RU5, and some SEP clients under its management are v14.3 RU8, Virus pattern update should be alright. But new features of v14.3 RU8 cannot be displayed and managed by SEPM v14.3 RU5 management console.