I think that being able to use your existing Solr webapp depends on two things: whether your current webapp is the same version required by the schema and solrconfig for the EADitor webapp (the EADitor version is 3.6 or 3.6.1) and whether your current webapp is configured for multicore or not. EADitor runs three cores for now: published finding aids, all finding aids in exist (used for the admin page), and vocabularies. If your current Solr installation is not already set up for multicore for DSpace, you may have to change settings in DSpace, which might be a lot of work. You could rename the EADitor solr war file from solr.war to eaditor-solr.war and run it separately. The URLs for Solr and for EADitor's public interface are now stored in a configuration file, not hard-coded. The config file is stored when you enter the admin interface for the first time; you will have the opportunity to change URLs.
The first thing you will want to do before going to the admin page, though, is to edit one config file which is stored in the filesystem, in the Tomcat webapp folder for orbeon in orbeon/WEB-INF/resources/app/eaditor. You'll want to edit exist-url.xml and change the <exist-url>http://localhost:8080/orbeon/exist/rest/db/
</exist-url> line to a different port number.
After changing the port, go to the admin page, as the directions at http://wiki.numismatics.org/eaditor:eaditor_first_run
state, and the admin page will detect that there is no file located at http://localhost:8180/orbeon/exist/rest/db/eaditor/config.xml
and the popup window to set up EADitor will appear.