People get to be mad (within reason) when a free service stops being free without prior warning. Because they too invested time and effort in a product, making it part of their life, only to have that wasted when terms change.
That is not to say, in any way, that you shouldn't charge - but don't expect users of a free application to change to a pay to use model en masse. Unless, of course, you don't have any competitors who do a similar thing for free...
That is not to say, in any way, that you shouldn't charge - but don't expect users of a free application to change to a pay to use model en masse. Unless, of course, you don't have any competitors who do a similar thing for free...