The upgrade plan was software every 2 years and hardware every 4, through the life of the F-35.
When the development price rose, the MOU had fixed investment from the partners and wasn't of a great concern, as the US stood the increase.. The software cost upgrades are another story and as far as I remember and unless it has been changed. The MOU has the software cost, divided by the number of tails you have. This rise affects all the partners equally as much per tail, as the US