No, not quite right. First of all, your real concern is the packages installed or removed from your system, which is what apt is designed to manage. As I understand it (and others here know better than me), a package installed on your system from sid will not be removed unless/until it is replaced by a newer or alternative package that makes it obsolete (except during the transition of a set of related packages, which is why we have "upgrade warnings").
Second, the acceptance of a package into testing does not remove it from sid. If it did, there would be no possibility of having a sid distribution that would work, because it would never be a complete set of packages for the OS plus a desktop.
However, you are not completely off your rocker. A month is not normally a problem, but if you have a system that hasn't been updated for a couple of years, like I do (for a good reason), then that system really cannot safely ever be upgraded -- there would be tons of breakage and it would take longer to fix than a clean new installation/configuration would take.
I hope that is a satisfactory answer.