The Prophets never deviated from Tawhid and fitrah as you've mentioned.
Until Allah's messages were revealed to them, they remained in the natural state of being under the best possible beliefs (religion without organization) that were pleasing to Allah, and after the message was revealed to them, the refined and corrected religion was revealed.
So basically, until the message was revealed to a Prophet of Allah, there was no true religion present FOR them to revert to.
I guess a good example would be Ibrahim AS. Ibrahim always believed in One Illah, even though such a religion did not exist anywhere around him. He looked at the religions around him and found them all to be wrong and stuck to his beliefs of One God until Allah revealed the message to him, wherin everything he had already believed was confirmed, and an organized religion with rules, guidelines, laws and codified etiquette was presented which refined the human nature and elevated it by affirming all that is intrinsically good and forbidding all that is harmful to the soul.
The people of Ibrahim AS, of course did not take kindly to this message and promptly began gathering some firewood...