It's a completely different system, that happens to support the same API/ABI, backed by long experience at Microsoft at making things backwards compatible (it's probably the biggest source of Microsoft's success).
Microsoft also put ~7 years into preparing the transition piece by piece, with probably the biggest impact point being Windows 2000 that provided significant host of features that compelled big (and loud in complaints) clients over, pulling a lot of software vendors along.
It's a completely different system, that happens to support the same API/ABI, backed by long experience at Microsoft at making things backwards compatible (it's probably the biggest source of Microsoft's success).
Microsoft also put ~7 years into preparing the transition piece by piece, with probably the biggest impact point being Windows 2000 that provided significant host of features that compelled big (and loud in complaints) clients over, pulling a lot of software vendors along.