Once you go to Bridge you will no longer need a Noah server. The key is to ensure that the computers you want to use Noah Bridge on are running Noah 4.13 - so you will install it locally.
If you have a Noah server and are migrating to Bridge it's your call as to whether you upgrade your Noah server - probably you would only do it if you have any security concerns.
All the Noah products that are installed on your central Noah server will be brought over to Bridge once you convert the license to an engine license.