AmigaOS4 will be able to execute existing Amiga software written for existing 68k
processors. AmigaOS4's kernel ("ExecSG") will recognise 68k code and automatically
start the (pretty good) JIT-emulator that is part of OS4.
However, this JIT emulator *only* emulates the CPU. Therefore, a standalone AmigaOne
will not be able to execute games or anything that requires Custom Chips
(DPaint, Scala, Brilliance). If you want to execute stuff that requires the Custom
Chips, you'll have to attach the A1200 to your AmigaOne.
The Amiga1200 CPU will be disabled - "your favourite AGA game" will be executed by
a JIT 68k emulator running on a G3/G4 CPU.
However, OS4 comes with several enhancements (Virtual Memory, Memory Protection),
that might brake old software that patches a lot of stuff (especially games - Amiga
games programmers didn't care very much about "clean" programming). So I wouldn't
expect many games to run without modifications. But the OS4 developers are in touch
with Bert Jahn (WHDLoad) to ensure that games patched for WHDLoad will run on an
AmigaOne with an A1200 attached.