my approach was always the same: i download the .t3x file of the extension version before i replace it with the current one from TER because i had no idea to bring back the older one if i needed it. so obviously that one bugged me quite a while:
how can i import an extension verion different to the currently released one from TER in the new Extension Manager?
and the solution, as always in TYPO3, was already there and just waited to be explored, as simple as effective.
right click on your found extension and you´ll see the option to get all versions of the extension to download from the TER
you enter the Extension Manager, go to Import Extensions, find your very extension and right click on it. the popup shows you the option to import versions of the extension, this bringing you all the allowed versions to import. A no-brainer, that most may know. I had to read the T3 security bulletin to learn it.
