iOS App Testing – How Can Test Users Access an Unpublished App?

A big problem with trying to be tricky is you never know when/if someone who purchased your app will run it again. I am assuming you don't have some sort of registration involved with your app where you have contact information of your users, that would be too easy.

Probably the best way:

• Introduce a new version that is free and keep the paid version. Some people might prefer to buy it outright in the future. Downside is it splits your "popularity" between two copies, but upside you get more hits in search results. You may want to differentiate them by name somehow, but if the free one upgrades to full functionality then avoid calling it "free" or "lite". While you are maintaining two apps, they can share 99% of the same code.

Every other solution I can think of would either be a lot of work, or have the potential to leave a lot of users out.

If you own 100% of the code in your application, (like in the iRail example you linked to) then you can dual-license the code: one for AppStore and one for everybody else.

If you don't own 100% of the (i.e. you make use of GPL third-party libraries) then you also need to get permission of those copyright-holders, and you need to get a new license from them before you can put it on the AppStore.

Note that not all open source licenses have the same restrictions as the GPL. I believe BSD, MIT and some other licenses would be compatible. (but IANAL)