Refund at a pro-rated rate, be frank with users and tell them the situation and why you have to raise prices - then raise prices.

I doubt users would have revolted had the subscription gone up to $19.95 or even $29.95 a year. Heck, my hiking app is more than that annually, and I barely use it.

I remain convinced Apollo had options, but the creator was convinced it was an impossible situation. It was not. There was an outpouring of support and people willing to spend money on an app they saw immense personal value in.