There is very little as destructive as a new idea. That may seem counter intuitive at first, but trust me. Over the course of my life I've seen new ideas kill more characters, games, and stories than I care to count. They have this weird siren's song that lures in the unwary, and then - before you know it - you're breaking free from what idea you have in progress in order to start this new thing. I mean, what chance does the old idea have? It's older, in practice. You've already had that fun.
Today I want to talk about that, and how to defend yourself from those new ideas - while still keeping them in case you have time for them later.
Recognize It For What It Is
The first step to handling a new idea is to recognize it for what it is. New ideas are very alluring because they're new and fresh. They haven't been implemented, they haven't been used, and they haven't been tested. This means you haven't found the flaws in them. You can't see the pitfalls. They appear flawless, but that's only an illusion. Once put in practice the new idea will reveal those flaws - and sometimes they're more numerous and problematic than what you have right now.
Resolve to See Things Through
The second step is to have the resolve to see things through. Whatever you're doing right now, you did it for a reason. Remember what the idea was. Remember why that idea is fun. Play into it. If it is a campaign you're running, remember that you have all your players who are probably having a god amount of fun. If you're a player, remember that your fellow players are likely counting on your character for various things, and that you stand to lose more than gain by swapping out characters in about 95% of situations.
Write It Down
So you're aware the idea's perfection is probably illusion, and you want to finish your current thing, but this new idea won't go away? Write it down. Don't flesh it out. Don't feed it your mental energy to develop it further, but jot it down, and put it somewhere you can find it later. Often by writing down the idea - or just a quick note about it - you'll clear it out of your head. Why? Because you wrote it down and our minds know that we write things down to not forget them, so once it is written down we don't have to keep it in mind.
The important thing here though is to not put development time in. The more you feed the idea - especially in initial stages - the more it can take hold. Even when written down.
But You're Not Having Fun
Changing gears. What if you're not having fun with the current idea? You've tried remembering what you saw in your original PC and it's not working. You hate your campaign and the idea of running another session stresses you out? Well, in those cases the new idea isn't the problem is it?
If you're not having fun with your game, or your character, you owe it to yourself and your group to discuss it. As a player, talk to the GM about not having fun with the game. As the GM, talk to your players about how you're not enjoying the campaign. Maybe it's something that can be resolved. Maybe it's an easy fix. Maybe it just requires a bit more screentime, or a little less murder-hoboing or something. Give the group the chance to fix it.
If they can't? Well, maybe then it's time to swap out for the new idea. If you're not having fun with your game, or your character, you shouldn't feel obligated to keep them around.
Just remember, if this becomes a recurring problem, it might not be the character ot type of game that is the problem.