HTC formally announced its latest handset today, a version of the One M8 running Windows Phone 8.1 rather than Android KitKat. The existence of the Phone made possible by changes made by Microsoft specifically for this reason. It's HTC's first Windows Phone since the 8X (and follows the precedent set by that handset for awkward naming of devices). and the first phone to give customers the choice of OS on identical hardware.
The One M8 is a highly praised handset but that in itself isn't going to present a reason for buying the Windows Phone version - especially when the Android one has better app selection and makes better use of the unique Dot Cover.
No the importance of this handset is that it demonstrates how easy it is for OEMs to convert existing Android hardware to Windows Phone.
And whilst there is no evidence of either Google or Microsoft allowing dual booting of handsets between the OSs, I'm sure some bright spark will deliver that sooner rather than later.
In terms of getting Windows Phone into users hands it might be exactly the thing that Microsoft is hoping for.