Hi there,
first you should make a staging site, so you can make those changes without affecting the live site. On the staging site:
1. Remove any custom functions or Additional CSS.
2. Remove any GP Elements that you may have used and won’t now require.
3. In Appearance > GeneratePress –> Reset the settings.
This should take you back to a fairly clean install.
4. Now import Marketer.
Update logos, menus, widgets etc. as your current ones will be swapped by those from marketer.
Once happy you can push the staging to live.