MSI in a recent MSI Insider briefing showed that its 400-series motherboards are planned to support AMD's upcoming 'Zen 3' microarchitecture. This MSI Insider video also showed that 300-series boards aren't expected to support the new microarchitecture. AMD has stated that they will be sticking with the AM4 socket but will be changing the chipset to ensure the maximum performance. Since AMD is changing the chipset, some chipsets may not offer support for older CPUs or older microarchitectures, even if the CPU does fit inside the socket of the motherboard.
The recent MSI Insider showed a slide that showcases that the X470 and B450-based motherboards will need an update to support the 'Zen 3' processors
In the MSI Insider presentation, there was a background slide which showcased which processors and which motherboards were compatible. Among the motherboards were A320, B350, X370, B450, X470, B550, and the X570 chipset-based products. This slide also showed how the X570 and B550 Motherboards were incompatible with the Athlon processors, and the 1st generation Ryzen processors while the newer B550 motherboards were also not compatible with the 2nd generation Ryzen processors in addition to the 1st generation and the Athlon processors.
For the maximum compatibility, the 400-series motherboards offer compatibility with everything from the Athlon Processors to the upcoming 'Zen 3'-based processors. Although to gain support for the upcoming 'Zen 3' microarchitecture, users will have to update the BIOS with a Selective Beta Bios Update. As stated by Michiel Berkhout, this update could potentially cause the motherboard to no longer support older processors.
The 300-series motherboards will not offer support for the 'Zen 3' microarchitecture, according to the slide. This means that any user still holding on to a 300-series motherboard will need to buy a new motherboard to support Ryzen 3rd Gen processors and any upcoming processors that utilize the 'Zen 3' microarchitecture.
The hosts to the MSI Insider, Eric Van Beurden, and Michiel Berkhout made an interesting point that when users flash their BIOS to offer support for these motherboards may lose support for the older Zen microarchitecture. This means that if a user buys a 'Zen 3' microarchitecture based CPU and flashes his BIOS to allow the motherboard to support that new CPU. The user may not be able to connect an older AMD CPU into that AM4 socket.