Pub 100: Pre-Publish 97 & Store Pets Slots Updating
"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.