Most of the people at free concerts are there because it's free, while the people at pay concerts are there because they are willing to pay to see a particular artist. If you switch to a pay series, you basically have to start building your audience up again from scratch--

As an alternative, it might be possible to alternate the free concerts with special low price concerts from high profile peformers who wouldn't otherwise be booked there--even still, it would have to be done so that people felt like they were getting something new, as opposed to being charged for something that used to be free--