NO, many of these games will play through strait standard DirectX emulation without any(or little) required manual work, some games will need to be recompiled by a compiler but most of the work will be done by the compiler NOT a human, yes the compiler will not be perfect but I'm betting ALLOT of the arcade game code is ALLOT more portable than you think.
This is Microsoft bread and butter were talking about here, if arcade\downloadable games are not "portable" to Xbox720 Microsoft loses ALL CREDIBILITY when it comes to downloadable content distributed via Xbox Live.
So, MS has developed an AI compiler that requires no human intervention? Awesome, I'll tell my team of .NET devs they're all fired.