First off, there hasn't been any announcement, heck not even a single rumor, about a 64 bit powered Windows Phone as of yet. Secondly, Microsoft has a huge tradition in providing support for legacy code. If I remember correctly the last pieces of 8bit supporting code vanished from Windows with Windows 2000.
Good thing about 64 bit systems is downwards compatibility.They can easily run 32bit applications. Just not the other way round.