I think most of us have gone through the same thing when we migrated, either to a new computer or from SONAR to Cakewalk. Often, when you're putting a new system together you'll be motivated to better-organize the new setup. Consequently, plugins might end up in a different folder or sample libraries on a different drive. These will result in missing-plugin messages, failures to load libraries or lost configurations.
The missing-plugin issues are
mostly easy to resolve. Pull up a project and make note of which plugins it couldn't find, then close the project without saving. Manually locate each missing plugin and add its path to the VST scan search path list. If the plugin is truly gone, you may have to re-install it.
Last time I migrated to a new system it was because my computer had been stolen. That was a mess, and some things just had to be given up. The time before that, though, I was able to have both the old and new systems running side-by-side and networked, and that made the transition much easier.
For example, many effects and instruments were unable to restore their project settings. So I went back to the old machine and saved all my Ozone settings and Kontakt instruments as presets (named for the project they were used in), transferred those presets to the new box, and referenced them in the moved projects.