I had even more embarrassing auto-corrects back when I was dating a girl into sext-messaging.
I never had this option because I always memorised all of my dealers' numbers. As well, you want to change your number so that they can't call you.
I think it's a good idea, but like others have said you have to do it once you are truly ready. Losing a private connection and then going on a wild goose chase for heroin after your first craving is a quick way to land yourself in some legal trouble (or worse).