I don't normally delve into discussions on alignment but...
Alignment is not a straightjacket (seriously, go back and reread paragraph five under "Alignment" on page 103 of the Player's Handbook - assuming this is a 3.5 game). Characters can (and do) take actions that are outside of what is considered normal for their particular alignment. Alignment represents a creature's
general moral and personal attitudes.
I usually don't play with groups that worry about your alignment. its more of your good you can't go around killing farmers cause your bored or your evil no you will not help those ppl burning in that church. so what alignment would work better for how im playing this char. LN N. can not be evil so if thats the case then i will have to change some character concepts.
I bolded a portion above. Being evil DOES NOT hinder you from assisting people burning in a church. Again, reread the section in the Player's Handbook on Alignment. It is a general attitude, not a strightjacket. There are many reasons that an evil character could further his own agenda by assisting the people burning in the church.
As to the question of: what alignment are you playing? That isn't answerable by any of us. We do not know the details of how your character has acted over a long enough time frame. However, I think that the specific act of forcably changing an NPC's alignment
is an evil act. And it would be ruled so in
my campaign. It
may also be an illegal act as well, depending on where and how it is done.
Assuming you do go ahead with changing this NPC's alignment then there is a practical consideration that you should think through. Changing the alignment does not mean that this NPC will suddenly become your ally. Sure, you have changed his general moral outlook, but you are still left with a character that his own unique agenda that he will pursue. Barring the use of
charm spells, there is no guaranty that his agenda will be inline with that of the party.