Raise Rate on Supporter Membership or offer discount code?

I have a few members on my site that currently pay $40 per month on a recurring subscription. I a want to raise this to $50 per month for people from now on. If I change it in the Support settings will old members get charged the new $50 rate?

Also on that note can I offer a $10 discount code anywhere? ie paypal, amazon, or in the supporter plugin?