Tarosill: I've only experienced pitcher changes a few times, but I've never been e-mailed, but maybe some books do so? Some books, even if they don't state the starting pitcher, will void when there's a pitcher change. It's worth taking a look at the rules for the books you're using to check their position on it. I've looked on Betclick's site, and they say the following:
"Baseball
All games must go 9 innings (8 ½ if the home team is leading). All games must start and be completed within 48hours of the specified start time or bets will be void. If a game is cancelled or abandoned before 9 innings are completed (8 ½ if the home team is leading) then bets will be voided, with the exception of bets that may already settled e.g. ‘1st team to score’. Both named pitchers must start. All bets placed prior to a pitcher change will be voided. The ‘Total Runs’ is the combined total runs of both teams at the end of the game. All bets include extra innings."
I agree this rule is confusing given that they don't state the pitchers, but I have seen it at other books, mainly European ones. They must be going off the team's announced pitchers as "named", but it does mean you could get caught out if you place a bet at around the time the team changes the pitcher as you wouldn't know for sure whether your bet would stand or not.
More experienced bettors can probably offer more insight than me on how common this is.