My original flight was coming up, so I had to change the reservation to a future date - as I don't know yet when I want to travel, I looked for any destination that had as closely as possible a price as the original booking. Here's the original value of the booking:

I then changed to these flights:

So the outbound flight was EUR 7.20 more than the original outbound flight, and the new inbound was EUR 2.60 cheaper. Ryanair made me pay the EUR 7.20 fare difference for the outbound, and did not refund the EUR 2.60

When entering the "change booking" screen after completing this change, this is what is shown:



So, it looks like you forgo any amounts resulting from changing to a lower fare

Overall, this makes using the "FlexiPlus" fare in a flexible fashion fairly complicated.