I don't think that any transfer can seriously change the chances of the teams (unless of course it's Haaland + Mbappe, but they hardly want Chelsea plus they can no longer play for another club in the Champions League this season). Maybe we will see Ronaldo's move that has been talked about so much? He's not showing anything special this season, but he has a certain Champions League magic associated with him.
Bringing in players who are already registered in the Champions League with other clubs cannot help Chelsea, they can only play in the domestic league, while the Champions League must wait for next season. Ronaldo is very likely to be played in the Champions League because the club that Ronaldo defended before did not qualify for the Champions League. This situation can be used by Chelsea to increase their strength to compete in the Champions League. Ronaldo really can't do much and isn't as sharp as he used to be, but his experience and skills can help Chelsea go a step further.
I was not aware about this law of UEFA because if they have anything like this surely it's going to help Cristiano Ronaldo and Chelsea for having better result in this season and with this option most chances in January window we will be having him into Chelsea without any doubt because right now Graham Potter is looking for the player like him who give them good performance and quality so with the arrival of Ronaldo into Chelsea they can do better things.
Even too many speculations about his career and age factor but still we can expect better performance from him because he has already done good for the Manchester United, and he was top scorer in last season but sadly having not good playing time under new coach Ten Haq which create problems for the both, and now he is out of the club.