BEAM is still present but HiPE interfaces with it using some specific ABI that allows mixing AOT compiled native modules (just machine code) and the BEAM register machine. I'm not sure if there are new caveats as I regard HiPE as a compatibility feature rather than something under active development (I may be wrong but it seems to fail to compile all of OTP's Erlang code now).

The overhead of that context switch is a bit high but it allows the code loading facilities of the BEAM to reload HiPE compiled modules. This works because all processes yield to the scheduler, which acts as a kind of code-swap safe-point. The usual module-local vs module-remote call rules apply here when old versions are purged.