You can train up to PPL/IR on a M-1 visa. As soon as you have a CPL it's over. You're apparantly only allowed to have a 100 hrs, but I can't find any official reference to that. When I got a J-1 I had 190 hrs already and nobody even asked about them.
However it's not a certainty that you get a J-1 after you had a M-1, I know there where some people who couldn't get a J-1 anymore. (Don't know the reason for that)
One thing you have to leave the country to get a J-1 and quite frankly you have to be really glad when you're in. Getting a visa doesn't mean anything until you passed immigration at the airport, they can decide to send you back like that.