In general, whenever updates are released involving the Email Service, older versions installed will continue to work as they always have. You will need to keep in mind that it will not be able to process any notifications during the 4 hour upgrade window as all applications will be offline. It might be advisable to pause it during that time, or remember that any emails will be sent to the Unsuccessful folder and will need to be moved to the Inbox afterwards.
The new changes to the service will be available to you with a new download of the service, which you can install at any point after the 9.3 release. It does not have to be within any certain time frame after 9.3 is released, you just will not be able to take advantage of the new features without an updated install.
I hope this has addressed all of your questions on this. If there is anything else we can clarify for you, please let me know.