Sigils can do a lot of things. However if someone really believes in something I would think it to be hard to change it with a sigil. Basically you would be planting the idea in their mind that their faith is not right for them. But if they believe that to be false it would not take. Why would you want to change what they believe in the first place? Does not change who they are ;)
It's possible but very unlikely. Since sigels are in essence a magical manifestation and promotion of ones will, it will have to compete with the will and faith of that persons religion. However as previously stated it can implant the idea of their religion being false or the desire for change. The sigel could still be effective if followed up my other magic and spells and act as a sort of primer. But I have to agree on it's own against a strong faith or belief, it will probably not be enough.
Belief has a lot of will power behind it, which is why many incorporate their spirituality with magick--it fortifies it. The only thing that can cause someone's faith to waiver is doubt. But that can very well backfire because generally when someone doubts their faith, they just stop believing in everything all together. It's so ingrained in their mind (usually by family and spiritual leaders) that their religion is the only true religion.
The only people I have ever "changed" the religion of were those that already doubted their own and were interested in mine.
But that is within my own experience. I can't say I've ever desired to convert someone. Christians generally do that and some Satanists (ironicaly). The other religions (including pagan religions) are not like that.