I have multiple sites with different content on them, I found a better template to use, is there an easy way to use the new template and copy the content over to it? Can I keep the content on my current site but change the site template without losing all of my content?
You cannot change a template once you have created the site. You can either make the desired changes to the existing site (new lists, libraries, page configuration) or you can create a new site with the desired template and then move existing content. If it is document content you can use the Explorer view to cut and past documents or use the send to feature to move the content. It is also possible to use custom or 3rd party tools to copy the documents and content from a source to a destination site. This may also make it possible to translate any of the meta-data as needed.
I just want to clarify before I answer. When you say "Site Template", are you refering to a file with an .wsp extension that lives in the solution gallery?
If that is indeed what you mean, you're going to have a bit of work to change an existing site template. You have to backup your database, save all your subsites as site templates, note your permissions, delete all of the sites, then recreate the site using the template you want, then recreate all it's subsites using your subsite site templates with the right permissions on all.
Not an easy task.