In order to replicate with each other both TimesTen databases must be using the same character set. Hence it is not possible to perform an online upgrade that includes changing the database character set. To do that you will need to take some downtime.
TIMESTEN8 character set is a binary 8-bit clean character set. It is not really a 'character set' as such but rather it maintains the behaviour of older, non-NLS capable, TimesTen releases. It is not possible to say therefore which of the many Oracle compliant character sets might be best for you; that depends on the actual data that you have stored and how you intend to use the data. None of the NLS character sets provide exactly the same behaviour as TIMESTEN8.
Thanks for your response. Is it okay for systems to run permanently with character set TIMESTEN8 for upgraded systems? Or do we have to move to one of the other character set. I'm trying to avoid any downtime if possible.
At the moment there are no plans to remove/deprecate TIMESTEN8 but I cannot of course make any guarantees about what may happen in the future.