Setup Extras testing, install a few packages and then disable Extras Testing. Probably if immediate damage, slight. But then the software is promoted to Extras, you might have some problems. I don't know how the Debian software installation software actually it's files.

You might have to uninstall the older versions before you can cleanly installed the promoted versions. (EDIT: In fact that would be my advice. Uninstall the old versions first. Don't let the Appllication Manager install the updates when the become available.) At worse, some minor command line cleanup as root might be required (which can be dangerous all on it's own).