Flights changed because of COVIT-19 and now cancelled by FlySafair

We had 2 booked flights.

1. ###### Original booking for 11 April 2020 George to OR Tambo. We changed that on 31/03/2020 to 24/07/2020. No problems yet with that booking.

2. ###### Original booking for 16 April 2020 OR Tambo to George. We changed that booking on 01/04/2020 to 26/07/2020. Got messages stating that flight was cancelled. Got 2 separate vouchers covering all the cost for the said cancelled flight. Why is flight 1 still in tact but flight 2 cancelled?

I checked on website and could make a booking for the "cancelled" flight. It only accepts 1 voucher and the cost of that flight is also now more expensive AFTER we already paid the difference between the flights.

Please explain