the Star Wars timeline, particularly in Star Wars' animated shows.
These updates to Dooku's story provided fascinating new insights into his experiences being Master Yoda's Jedi Padawan and Qui-Gon Jinn's Jedi Master, in addition to his complex relationship with his assassin, Asajj Ventress. Despite some more recent appearances in Star Wars, however, a number of mysteries remain regarding Count Dooku and his path to becoming a Sith Lord. Certain retcons in particular have raised brand-new questions about when exactly Dooku first became a candidate to be Sidious' new apprentice.
Tales Of The Jedi Revealed Dooku Became Palpatine's Apprentice Immediately After TPM
Tales Of The Jedi Provided Brand-New Details Of Count Dooku's Timeline
Star Wars: Tales of the Jedi followed two distinct three-episode arcs, one for Ahsoka Tano and the other for Count Dooku. Count Dooku's arc proved particularly interesting, as little has been shown of Dooku's story on screen, particularly predating Star Wars: Episode II - Attack of the Clones. Among the reveals from Tales of the Jedi was the fact that Count Dooku became Darth Sidious' Sith apprentice just after Star Wars: Episode I - The Phantom Menace.
In a sense, this is unsurprising. Palpatine was always several steps ahead of everyone around him, even someone like Dooku who believed he understood Palpatine and his plans. Palpatine also made it abundantly clear throughout the prequel trilogy how willing he was to replace his apprentices. That he filled this gap quickly makes perfect sense for Palpatine's character.
Nevertheless, this is an interesting update to Dooku's Star Wars story. It was already known that Dooku had been a Jedi who left the Order yet retained the respect of the Jedi Council; this is why Mace Windu could not believe Dooku was behind the assassination attempts on Pé in Attack of the Clones. What was less known was the exact timing of Dooku's turn to the dark side and allegiance to Darth Sidious as his Sith apprentice.

Star Wars' Latest Asajj Ventress Reveal Finally Explains When She First ed Count Dooku, & Sets Up A Massive Rule Of Two Conflict
A brand-new Star Wars book reveals that Count Dooku had a very different plan for Asajj Ventress, and it completely broke the Sith Rule of Two.
Dooku Was Already Operating Under The Name "Tyranus" Before TPM
Dooku's Sith Name Raises Questions About This Timeline
What complicates the timeline introduced in Tales of the Jedi is the confirmation from Star Wars: The Clone Wars that Count Dooku was already using Tyranus—his Sith name—before The Phantom Menace. In The Clone Wars, it was revealed that Dooku had betrayed Jedi Master Sifo-Dyas, the Jedi who felt the Republic needed an army to protect itself, and had him killed. He did so by hiring Lom Pyke to shoot down Sifo-Dyas' ship.
Dooku was using his Sith name before Darth Maul died and thus before he was Sidious' apprentice.
This assassination was commissioned by Dooku using the name Tyranus. In light of this, and given he immediately went to Coruscant afterward based on this Tales of the Jedi arc, it seems Dooku was using his Sith name before Darth Maul died and thus before he was Sidious' apprentice. Notably, Dooku did not seem to be using "Darth" Tyranus, but this nevertheless raises serious questions about when Dooku truly began serving Sidious and what that means for Palpatine's plans.
Was Dooku Already Competing To Become Palpatine's Sith Apprentice?
As Maul Himself Said, He Was Always Disposable In Palpatine's Larger Plan
Dooku using the name Tyranus while Darth Maul was still alive suggests that he may have already been in line to become Sidious' next apprentice. This would be in keeping with what Maul told Ahsoka Tano in The Clone Wars. In Sidious' larger plan, Anakin was always meant to fall to the dark side and become the new Sith apprentice; Dooku and Maul were expendable from the very beginning and never had a real chance. This makes it even more likely that Sidious had Dooku waiting in the wings long before Maul's demise.
Perhaps Sidious and Dooku had planned for Maul to be replaced even before his death, and Sidious had therefore already given Dooku his Sith name.
It also seems likely that Dooku knew of Maul's role as Sidious' apprentice and was aware that he was being positioned as his replacement. In fact, in Tales of the Jedi, Dooku knew Maul's name, suggesting Sidious had given him some degree of insight into his plot. Perhaps Sidious and Dooku had planned for Maul to be replaced even before his death, and Sidious had therefore already given Dooku his Sith name. This would have been wise strategically on Palpatine's part, as it would have encouraged trust and loyalty from Dooku.
In truth, this timeline still needs to be clarified. The exact timing surrounding Dooku's disillusionment with the Jedi Order, involvement in the creation of the clone army, and earlier arrangements with Palpatine are not entirely revealed, even now. Yet, these subtle details suggest that Count Dooku may have been picked as Darth Sidious' next Sith apprentice long before Darth Maul died.