Yes, once enabled, it is always enabled no matter the version. However note that pushing Major Versions may involve reaching out to Salesforce twice. Here's what worked for me:
If you already have Push Upgrades, you will still need to open a case in the partner portal under the category AppExchange & Feature Requests > Feature/License Request in Trial or DE Org:

from Setup > Packages > [Your Package] go to Versions > Push Upgrades as usual:
then hit Schedule Push Upgrade

note the subtle change to the selection dialogue, which will now show major versions!

Enjoy! Remember, with great power comes great responsibility.