[eddiedunn]

Kickstarter campaigns often charge people extra money for getting access to Alpha and Beta builds. People are impatient, many want stuff sooner rather than later, so they pay for it.

You could offer a modified version of this; allow premium customers access to new features earlier, but roll out the updates to your free subscribers eventually as well. After a delay of 3 months perhaps?

It will gently encourage people to upgrade, but those who don't want to pay won't feel cheated as they will get the features eventually as well.