When will Matter products be released?
The Matter standard, and all the products that use it, might be released in Winter 2022. Might is the key word here: Matter was originally meant to be released in 2019 and has been delayed 3 times since then. If a Winter 2022 release is achieved, products will be available in significant volumes in 2023.
Can existing smart home devices be upgraded to Matter?
In theory, all devices with a 2.4 GHz wireless antenna which support the IEEE 802.15.4 protocol can be upgraded to support Thread and, therefore, Matter. Zigbee, ISA100.11a, WirelessHART, MiWi, 6LoWPAN, and SNAP all run on that protocol so, in theory, devices using them can be upgraded to Matter. In the real world, it's going to be very different, however. The underlying code for Matter is rather large (some would argue bloated). That means that the vast majority of smart home products already made cannot host it - they simply don't have enough flash memory. Further, there has to be a willingness from the manufactures of said smart home products to support Matter - they have to want to invest their time, money, and resources into creating and certifying new firmware. Simply put: most won't and most smart home devices already produced won't come to support Matter.