The New Legacy?
It's quite possible that MS will manage to arrange things so that recompilation of source code will probably produce a working ARM version of an existing x86 application. It's different to Mac's migration from PPC to x86 - there was an endianness change.
But with x86 to ARM there isn't, and that makes a pretty big difference to the porting task. All MS really need to do is to ensure that C structures (or the equivalent for your chosen language) are packed in memory in the same way and that's quite simple to achieve. Sure, there'll be testing to be done, but I'd put a whole £0.05 on that testing being confirmatory rather than fault finding, provided MS get it right.
MS have already shown some good evidence for it really being that simple. They showed Office 10 printing to an Epson printer. I don't know about Office 10 (.NET?), but the printer driver was simply recompiled and just worked. If a recompiled driver just worked OK, there's good chances for applications too.
There will be an emphasis on software providers actually bothering to recompile their applications. But if it really is that easy then open public beta testing will probably be an attractive way of keeping porting costs down.