How to ensure Time-Machine can rollback to a specific-date backup? (not deleting it)
There is no way of marking a TM snapshot as being particularly important.

However, as long as the TM disk isn't full, then TM should not normally delete the snapshot that accounts for a complete OS change. (Assuming that you had the old OS on your disk for more than 1 month.)

TM will start deleting the oldest snapshots if the drive is full, to make space for the newest ones. 

If you want to make absolutely certain, you would do well to use an app like SuperDuper! or CarbonCopyCloner to make a clone disk image, which you can restore at a later time. (But you needed to do this before upgrading.)

Bear in mind that when you roll the whole disk back, you will lose any changes to your data made since the rollback date. IMAP servers will handle changes to email, but documents and other files will need to be restored by other means.