"Pet Training: Pre-Publish 97 & Store pets that start Animal Training may have their pet slots updated based on their pet strength and will be given a warning before proceeding. Pets will no longer be able to start training if it will result in the pet being over the max cap."
This is a vague statement that when read at face value seems to imply that all "legacy" pets will be updated due to their high intensity value. However, upon further examining and testing, there is only a few animals that this particular note is directed at.
So let's get this out of the way. Your prepatch Nightmares, Bane Dragons, White Wyrms, Dreadmares and Bake Kitsune will be fine. As long as your prepatch animal was not expoited, nothing will change. You will be able to train it like normal.
If you have an old legacy 3 slot dragon, the ones who have been untrainable since patch 99, if you press the little blue button and attempt to train it, it will give you a warning one time only:
What that means is the moment you pick "Yes" your dragon will without any training go from three control slots to four. Making it exactly like the current spawning dragons. There will be no difference.
If you press cancel and then hit the blue training button again, your pet will immediately be updated to 4 slots without warning. You only get that warning the first time you press that button. You will never get that warning again, not even 6 months later if you decide to press the blue button.
If you play on the enhanced client with a custom UI like Pincos, you will not receive any warning, it will immediately go up to 4 slots.
There is one more nonexploited pet that will get that warning and immediately jump up a slot if you try to initiate training, the Windrunner vanity pet.
The Windrunner will go from being 1 slot to immediately being 2 slot with updated control requirements if you try to initiate training.
Again, you will only get one warning.