The G36 will not be replaced entirely, though. The MK 556 will become the frontline service weapon, replacing in particular the G36A0 and G36A1. Several tens of thousands of G36's acquired for the War in Afghanistan will remain in service. They're only a few years old and were fitted with aluminium hand guards and reinforced receivers addressing the (factual) quality issues present in some older weapons anyway.
It is my understanding they plan with up to 120,000 MK 556's, some 40,000 G36's and something in the vicinity of 5,000 G38/G95's (HK416) in special forces use. Additionally, something north of 100,000 G3's remain in storage.
The MK 556's introduction process will supposedly take place in a staggered manner, with about 20,000 annual deliveries beginning in 2023. In other words, the G36 is going to continue to serve till 2030 at the very least. Oddly enough, the new rifle's designation is still unclear. The next open slot is G39 – though it might've been spent on a contender during the test campaign –, and they recently made a leap all the way up to G95 for some reason.