No, because the OS would immediately crash having lost half the logical processors it was using. This is why this is usually a BIOS setting. There are other issues such as the cooling profiles being predicated on the CPU configuration which cannot change in most BIOS/OS's after boot. But from a purely hardware perspective, there is no reason a Zen2 CPU cannot hotflip SMT support. It just needs to be in an environment expecting that possibility - like a custom OS
https://community.amd.com/community/gaming/blog/2017/11/27/asdasd
Not saying anything about consoles but Precision Boost 2 allow variable frequency amongst individual cores depending on workload and the number of threads in flight.