[GraemeJ]

...... I've angered a number of users by charging for MobileSheetsPro instead of releasing it as a free update.

Mike, disregard them - they are the same people who think downloading music, software or videos should not be paid for.  The concept of intellectual copyright, or simple hard graft and the labourer being worthy of his hire, is totally alien to them.  In your shoes, if anyone complained to me, I'd charge them double, if only for the aggravation factor. 

I'd like to find anyone who has a cogent argument for making this a free release.  It's hardly an 'upgrade', more of a complete re-write, with many additional features.  Real musicians fully appreciate the work you have put into this release and are not going to begrudge you one cent of the price.  In fact, many are already saying you are not charging enough!  You have enough people here who will support you and your efforts - tell the others to take a running jump.