2015 - you sold v1 for $5.00
2017 - you start selling v2.
You sell v1 for $5.00 but in the description tell everyone not to buy it.
You sell v2 for $5.00 but direct people who have already bought v1 to buy the bundle of v1 and v2 that you price at $8. Anyone who has already bought v1 can buy the bundle for $3.
N * (N + 1) / 2
Not very satisfactory, but very prevalent.
And, I'd say, given the low price of apps, it might be just right to not offer any special discount for an upgrade. You could always reward older users some other way.