Some characters have been portrayed in their modern attires as well as others appearing in the show before their debut in the original continuity. The biggest key differences would be the portrayals of some characters, certain subplots being omitted, and more importantly the ending. The former was done most likely out of aesthetic choice and as a way to give nods to the series in general as it would not affect the plot in a major way.
The latter would affect the plot of the original continuity with the most minor being the subplots of Nina and Anna Williams as well Paul Phoenix. The former being out of cryogenic hibernation with Nina having no recollection of her memories and brainwashed by Ogre into eliminating Jin Kazama. Paul's case is that he would thought to be the winner of the tournament by beating Ogre and leave.
Xiaoyu's and Hworang's character in the retelling has been faithful but toned down to a merit where their interactions with Jin have been detailed to degree though not to the point where their relationship would fleshed out. Heihachi was relatively monotoned as it was already foreshadowed that he would be an antagonist through his ruthless nature and betrayal towards Jin. Any fans of the series would know immediately.
The biggest difference between the original continuity and the retelling would be the ending. in the original, it would take place inside of Ogre's temple where Paul, Jin, Hehachi, and Ogre would be the only characters involved as their fights were isolated from other characters. Paul, who thought to have won the tournament by beating Ogre left only for Jin to face Ogre in his true form and beat him and In Jin's ending, Hehachi would betray Jin by having the Tekken force gun him with him being the one to pull the last trigger. This would inadvertently awake the Devil Gene inside of Jin and wreck the Tekken force and slam Heihachi through a wall and fly away.
In the retelling, all fights took place in an arena. King would beat Paul and move onto the finals where Jin barely defeated him and won. Hehachi irate from a barely standing Jin's take on his tournament progression challenges him and loses to him as he kept his Kazama teachings. This would provoke an awakening from Orge and fights Jin. Jin won and bested his true form with the help of Paul, Hworang, and Xiaoyu. After the fight, Heihachi pulls a gun and shoots Jin with the other three witnessing. This would awaken Devil Jin and he would beat him to the ground only to stop at Hworang and Xiaoyu's request and flies away upon recognizing them. This would most likely done to cut time though this would actually cause a plot error as no one except the Mishimas should know about the Devil gene at that point of the story. Hworang who would remotely come to learn about it should not have learnt about it this early onto the story as Jin would wipe the floor with him unconscious in his Devil form in Tekken 5.
Overall all though the retelling may greatly affect the average series fan's experience of watching, this shouldn't ruined it as this is a retelling of Tekken 3's story as it heavily focuses on Jin's character development. The show could improve on some areas such as Jin's relations to other characters such as Jun Kazama, Hworang, and Xiaoyu as it could greatly detail how they interact outside of the ring and to increase the watch time as it was too short and half of an average anime season of 12 episodes.
