In Stefan Returns, Steve later improved the formula to limit the negative effects it had on his new personality, and re-dubbed the formula "Boss Sauce." He also invented a "transformation chamber," which allowed him to turn into Stefan for extended periods of time (as well as avoiding developing rashes in "personal areas"). This time, he was a more likeable person which Laura is pleased with. He changed into Stefan several times—even while dating Myra—but some circumstance caused him to change back into Steve each time. In the Season 6 two parter episode: To Be or Not To Be, Steve was stuck as Stefan for a while, after Carl was accidentally transformed into Carl Urkel, due to Myra's tampering with the transformation chamber to sabotage it and preventing Stefan. Late in the sixth season, Steve transformed into Stefan as part of an inventors competition at Walt Disney World in Florida; however, Laura sabotaged the transformation chamber to prevent him from turning back into Steve. During his extended stint as Stefan, he proposed to Laura in front of Cinderella Castle. Laura accepted, but their engagement was broken off when Myra appealed to Stefan and Laura revealed her sabotage. While everyone liked Stefan, Myra disliked him immensely, believing he was a joke to her and thought her "Steviekins" is perfect the way he is.
In the seventh season finale "Send in the Clones", Steve created a cloning machine. He came to believe it did not work though it did create another Steve. To clear up the situation, Laura proposed that one of the Steves be turned into the suave Stefan.
The permanent Stefan made several more appearances throughout the series and proposed to Laura again before the series finale. After weighing her choices in the flashback episode "Pop Goes the Question," Laura chose Steve over Stefan, apparently realizing that if she married Stefan and got everything she wanted from him, he'd end up reverting to the arrogant jerk that he was and treat Laura horribly, leading to divorce. He left and never reappeared again. It could be assumed that Stefan accepted their relationship and moved on with his life, after he learned from his mistake in the episode "Crazy For You" when he teamed up with Myra to stop their date. Before the events of Crazier For You, he felt guilty of stopping Steve from dating Laura and had told him the truth about it. Stefan also tells him that he also suspects that Myra was stalking and spying on him, which proves to be true when Urkel learns about the illegal surveillance gear in her room and confronts her for her actions.
If the show continued in its final season, it would've been revealed that Stefan, upon learning about Myra's intentions to stop Steve and Laura's wedding, attempts to stop her of course. It also take four tries before he was actually successful in getting her to leave them alone. He also takes the time to get to know Myrtle Mae Urkel, OGD and the rest of the Urkel Family before he faded away in Death of Stefan part 2, after Steve realizes that his cloning machine does not make "permanent" clones like he intended. After that Stefan's personality and appearance reintegrate itself back into Steve, giving him a more mature appearance and deeper voice.
Dr. Urkel and Mr. Cool (Debut)
We're Going To Disney World (Parts 1 & 2)
Send In The Clone (Now a real person)
Paris Vacation (Parts 1, 2 & 3)
Le Jour d'amour (mentioned in first vignette; appears in second vignette)
Crazy For You (Part 1)
Crazier For You (Part 2)
Pop Goes The Question (Final Appearence)