I don't think you're forced to change characters during missions, if you are they there's a design flaw somewhere since the AI should be programmed to 'snipe the bad guys' or 'fly the chopper' or 'escape the cops' too otherwise playing certain characters in certain missions would be totally broken. You might need to change characters in order to play certain missions and progress through the story, but I think that's it. I know that some missions are unlocked by reaching a certain amount of money which you'll then presumably spend on previsions for heists and crap.
