Different builds. You would have to redownload everything. Just like beta installs don't work with full game releases
Technically every time you update most modern games you're getting a different build to a degree. It's rare for games these days to do typical "patching". Most modifications are to existing core data as opposed to additional data, at least outside of content updates.
The difference in applications like Uplay, Steam, etc. is what Repos and Game/App IDs the product is tied to. In this case the upcoming test will be using the same Game ID as the Private Beta (Alpha, Previous Tech Tests, etc.). We compare the build (files) you have on your machine and then diff them to find what has changed and what needs updating. In essence you'll just patch the Private Beta files if you still have them... (/u/nethfel was on the right train of thought)
The Beta and full release will be separate though so you'll need to re-download the game come release. You can still move the beta build and get the file diffing to work if you really want to, but it's typically not recommended. If you have the band...
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