To test existing XML, download locally, and then you can find under Form Options > Preview. I usually set versioning in the Form Options to not upgrade for this test as well, so if the schema changes involve anything that might cause a problem for existing data, I'll get an error when I preview. You can find more instructions here: https://www.infopathdev.com/blogs/hilary/archive/2009/03/13/first-do-no-harm.aspx
For setting up the library to use content types, first, get a copy of your currently published template (from the Library settings > advanced) and then republish it as a content type. You need to walk through the full publishing wizard for that. Once that is done, in your library settings, again under advanced settings, change the library to use content types. Select your newly published content type and set it to be the default. Then, back in the library, find the Relink view. There should be a relink all button - that will link your existing XML to the new default content type. At this point, after you've verified everything is working as anticipated, you can publish your updated form as a content type. In the library content type settings, you can set the new form template as the default content type, and then going forward, the existing XML will open with the content type they were linked to, and the new button in the library will use the new form.