well- the test center is for developers only. So they can upload and test their apps. But it will not work for apps you did not build on your own, I assume.
I am not sure, but you should be able to install a desired (previous) version of an app tthrough the univention-app-installer. You might be able to append the version number.
Once you have installed a previous version you should be able to upgrade to the latest.
Give it a try