I have bought GlumBuster several times to support the developer. It is a truly wonderful and unique game, but hte website is sooooooo baaaad and he is not very good at mpromoting the game, so it remains obscure:http://cosmind.powweb.com/blog/
Regarding free crossgrading (Never heard that term before) between Pc/Mac.. thing is, we promised a mac versin waaaay back in the day, and it seems mean for people who have stuck with us all this time to expect them to pay twice for the same game.
From a business point of view it is defensible as well, as an initial push of people playing the mac port will basically act as a marketing wave, showing as many people as possible what this crazy game is all about, and it will be shown by many people who already know the game, thus showing off things they like. :-D
The first argument, the ethical one, seems enough to me though. I suspect our Mac sales will be more than healthy anyway, as the mac community is not well served with decent indie games.