I think there are a few things everyone suggesting a monetization scheme should consider. Here's a big one:

http://www.codinghorror.com/blog/archives/001201.html

That's the post where Jeff Atwood explains the 90% software piracy rate. I think it's one of the most illuminating posts he's ever made. If you close the source, you will probably immediately begin to experience piracy, and it will probably be at a rate around 90%. Make sure you have a solution for that problem in your monetization scheme. (This also disproves that notion that most people are willing to pay for software. I myself haven't paid for a piece of software in years [though I don't pirate ANYTHING either].)

Consider also the GPL, which clearly states that the rights it grants are irrevocable (section 2). Even if the source were closed for new versions (and if the source were closed, and this is absolutely vital, the GPLed parts would have to be rewritten since the GPL prevents inclusion of GPLed code in closed-source projects [section 5]), the old versions would still be open source. If the software is popular, I do not find it hard to imagine someone would fork it... or just keep using the old version.

Keep in mind the author's comment that the software has benefited from being open. Closing the source would also mean decreasing quality.

Finally, the goal of keeping your software hackable is noble. Hackable software is a gift to programmers everywhere. It's not unreasonable to want to continue offering this.