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Have your heir be tutored by a catholic. If he is too old (you need to start at age 6 to be 100% sure) just wait for next heir. If you don't have any catholics just invite catholic claimants to your court.
 
Have your heir be tutored by a catholic. If he is too old (you need to start at age 6 to be 100% sure) just wait for next heir. If you don't have any catholics just invite catholic claimants to your court.

It usually doesn't take long to have your heir changed into his tutors culture/religion. Even if he's 15 its worth a shot.
 
If your siblings are heretics they try to convert you so this might happen with orthodox people as well, although I've never seen it myself. Do you have any catholic siblings?
 
If they are a member of your court (and possibly realm)you should see an option on that character's diplomacy screen where you can demand they convert to your religion.
 
One way that works 100 % is to swear fealty to a higher ranked lord that has the religion you want to convert to.
You can than choose the decision to convert to his religion. You can also change your culture to his this way.

On the downside, you lose 500 prestige points and your independence, at least for some time...

Another way is to have a spouse with the religion you want to convert to, it might offer the chance to convert by an event.
 
If you have RoI:
Conquer a catholic province and relocate your capital to that province. In the intrigue menu you can change your religion.
 
It depends where the ruler is. Just raise your country levies and have your ruler as the commander and move him to the heretic (if you want to convert to heresy, same either way) region. There should be an event. Yes, this is in ROI.