Why was one of my email addresses upgraded to the new look but my other was not?
As near as I can tell, they are doing it on a per-account basis. Any secondary account you have is treated as separate for purposes of the updating. There is no telling when your other account might get migrated. It will just happen one day.
Can I go back to the old version somehow? (I really hate how the "upgrades" happen without warning or without my authorization).
No. Once you have been migrated to the new format, there is no way to revert to the old one.