Yes, adding another card type to your note type, or changing a note to a different note type does require a one-way sync. You got this warning when you tried to do it.
On the device where you made the change, you need to choose upload to AnkiWeb – then on all other devices, you need to choose download from AnkiWeb. Until you accept the one-way sync on each device, it’s still outstanding.
But whenever you’re using a device you haven’t used in a while, you’ll always know that you need to choose download, because there’s nothing special or new on that device.