How can I get an rpm version of openfire after I modified the code?
There is an ant target that will generate an RPM installer for you. You will also need to have the proper linux environment to build it.
If the modification to the code is minor do I need rpm at all? What do I need to do to upgrade openfire on my server to reflect the changes I made to the code on my local computer?
I guess that you are using Openfire in your own installation (i.e. not distributing it). You can use the provided RPM to install the server and then execute ant openfire to build a new version with your changes. You can then replace the lib/openfire.jar with the one that you built or you can replace the plugins/admin folder if you made some changes to the admin console.
Will I be able to upgrade next versions of openfire and repeat the process of modifying the source code?
Ideally, you should implement your functionality as a plugin so that you can keep updating the server and hook up your code in a decoupled way. If your changes are generic I can include them as part of Openfire and they will be official so no need to merge them with new versions. Otherwise, you will need to always use your build when updating the server.