Which odds will go down? PSG or Chelsea? If PSG I think the odds will remain the same more than 1.5 because if it is below that it will make bettors choose PSG because it has a very convincing chance of winning, with odds of more than 1.5 it will make bettors a little confused because as you said nothing is impossible in football anything can happen even clubs that are considered weaker can provide surprises so we can't underestimate Chelsea because Chelsea has also fought so far they have been much better this season.
This shouldn't be the first time that Chelsea are being considered the underdogs in finals but Chelsea have triumph more times against teams in their best prime. Bayern Munich in 2012, Manchester city in 2021, the prime of this two clubs were better than that of PSG but Chelsea won. Chelsea isn't going to go down without a fight in this final and if they were to lose it shouldn't be more than two gaols margins because Chelsea is definitely going to score for sure and might get up to two goals so unless PSG can score more than Chelsea, they aren't winning. This was probably the best outcome of this tournament, I wouldn't had wanted anyone else in the final if not this two clubs.