Doing it in ten years will depend on how converted you want them to be, how much people you want to convert and how far you're willing to go. While in ten years you can easily have a majority of people accept the new god, you can't convert all of them. Furthermore, you can't really guarantee they aren't just pretend-converted.
Fear works fast
Fear usually works well to manipulate people. Torture doesn't get reliable information because people will say anything to get it to stop. On the other hand, people will say anything to get it to stop. If your goal is to make them say your god is cool and theirs suck
, don't read any further.
You could also genocide unbelievers for similar fear-striking results. It may be a fast solution, but it may also reflect badly. Particularly if your religion features "you shall not kill" as one of its primary rules. People might have a moment of brilliance, put two-and-two together and start revolting because your religion is hypocritical, so be careful with the executions.
Lost in conversion
The problem you face here is how genuine people are about your religion. Someone with survival instinct may show up for mass like a good believer, and then just sit their praying to their old gods in their head with a smile, and you wouldn't see the difference.
Conversion is hard to force. It's hard to convince someone that the thing they believed for all of their lives is wrong. Forcing change on someone may have all sorts of reactions, like resentment and hate.
Therefore, while force and fear may have immediate positive results, it could backfire later. Like they say, who lives by the sword, gets their head chopped.
I may be going to be wildly anachronistic but bear with me.
What you are asking for is change. And the oldest trick in the book of change management is: make them think it's their idea.
Consider this: what if you just put your religion in the competitive space? Build new cool churches, and hold receptions with free food or whatever will get people to come. Your religion has an explanation for anything, you may hold a Q&A sesh where every question is awarded. You know you have the answers anyways, right?
Make them compare your religion with theirs, and then make it so the comparison favors yours. They may not all come to the conclusion your religion is better, but some will. And since they'll think they came up with that, they'll believe much harder than if you just point a gun at their head.
And the bonus is that it's a double whammy. Your religion will be there, projecting a positive image. Sometimes, you just have to repeat something until it becomes the accepted universal truth. That's what advertisement is, and it works. Your religion has to be there, it has to remind you that it exists, and it has to show that it is cool.
I wouldn't go as far as advertising your religion as a new disruptive spiritual paradigm in the religious headspace, that might be a bit too avant-garde, but telling people that your god did it enough might just convince them he actually did. At worst, it won't be that foreign god that came out of nowhere, it'll be just a normal part of their life they don't reject. Non-rejection is actually a pretty good result.
Okay, but you still can't convince everybody, so what do you do with the rest? Nothing. They'll die off eventually. The ultimate long con isn't to change the present, it's to change the future, and future starts with the kids.
That's why the Church
starts indoctrination early always welcomes children. Although I guess it could also be for the diddling, but that's another issue. Kids will believe anything: Santa Claus, God, Disney princesses, that life is fair and they'll all be equal, etc. That's why any good dictator has to be on point with education.
It's hard to convince someone that the thing they believed for all of their lives is wrong. And when that thing they believe is the thing you want them to believe, then it's a good thing.
The kids will become adult believers that will teach their kids that your god is the best. They may even convince their parents because what they learned at school must be true.
The side effect is you just created a religious dictatorship.