What? Christina is not an absolute monarch. She does not have the power to declare Sweden catholic. She has at her coronation given a konungaförsäkran (a coronation oath) in which she has sworn to uphold the sanctity of the Swedish lutheran church as enshrined by the Uppsala meeting in 1593.
If she breaks her coronation oath, the crown council will declare her unfit to rule and invite Karl Gustav to take the throne, which he immediately would. He already controls the army, so there's nothing stopping him. She would have no support whatsoever in such a move, and while she might be able to flee to Poland-Lithuania, she cannot contest Karl Gustav, who will call an estates parliament session which will confirm his ascension to the throne.
Kristina knew that she could not be catholic and Queen of Sweden at the same time, which is why she abdicated first and converted afterwards OTL.