If I reimport an application to a server will the existing constant values get o

If your application package contains the rule, it will be overwritten with the new value but you will have the older one versioned.
On the other hand, if you dont want the values to be overwritten, remove these rules & constants from your application package so they dont over-write. Alternatively, you can take an export of the rules and constants from the target system as a separate application and reimport them after you import your application so the values get overwritten once again with the correct values (You may want to consider the first approach as the preferred approach)