Yes, but you can't leave it like that forever - Steam needs to 'phone home' occasionally to authenticate game and DLC ownership.
But the 'bother' you refer to is putting in one password once. Is that really such a big deal?
Not really, given that the setup works and codes are made available. But I prefer to do it at my own initiative, when I know there are codes available, all potential bugs are fixed, etc., and I really can switch back to the old version immediately. Because it seems that Steams upgrades to the latest version automatically on Oct. 1, and only on that very same date Paradox provides the codes?
I would liked better if Paradox had deferred the automatic upgrade for a day or two after codes have been provided, and (even better) for it then to only apply to games for which codes haven't been entered for the chosen version.