You may come up with CA Top Secret r16.0 on one side of a shared security file with CA Top Secret r15.0 without issues. There is no need to reformat a security file simply to test base CA Top Secret r16.0. You can also reformat a CA Top Secret security file under r16.0 and share with an r15.0 system as long as you do not turn on any features within r16.0 that will cause security file structure changes.
The current CA Top Secret r16.0 features that would alter the security file format and, therefore, disallow sharing are:
B-66394 Increased User Record Size (ACID) to 1024K
E-11835 Store FACILITY Parameters On Security File
This will actually not be a problem but the facility parms will still need to be maintained on r15.0 separately
B-66429 AES 256-Bit Password Encryption
This feature is not currently available but will not be possible when sharing with r15.0
As each new feature is released it will be stated whether or not that feature will be available on in a shared environment with r15.0.