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When you're ready to switch to testing, I'd just switch your sources from unstable to testing, or adjust your preferences. You would also need to downgrade the packages that have an unstable version that is higher than the testing version from the unstable version to the testing version. This downgrading may or may not be easy to do, but I don't think that is anything else you can do. A separate question, which you have not asked, is whether this is a good idea. I don't believe it is. because for much of the time till late in the testing cycle (or so I have heard) unstable is actually more reliable than testing.

If you want to ask about whether your procedure is a good idea, you could add that question to your current question, or write a separate question. I think the former would be reasonable.

To downgrade an individual package, the following works to a first approximation

apt-get install pkgname/release

In the case mentioned here, release=testing. You might need to also force a downgrade for additional packages, depending on the dependencies. You can specify them on the command line as additional arguments, namely

apt-get install pkgname1/release pkgname2/release ...