Merge pull request MicrosoftDocs#1062 from sptramer/migration/az-rationale

Update the 'introducing' article with detailed rationale information.

Author: sptramer

docs-conceptual/azps-2.0.0/new-azureps-module-az.md

@@ -14,12 +14,35 @@ Starting in December 2018, the Azure PowerShell Az module is in general release
 PowerShell module for interacting with Azure. Az offers shorter commands, improved stability, and
 cross-platform support. Az also offers feature parity and an easy migration path from AzureRM.
 
-Az uses the .NET Standard library, which means it runs on PowerShell 5 and PowerShell 6.
-Since PowerShell 6 can run on Linux, macOS, and Windows, Azure PowerShell is now available for all platforms.
-Using .NET Standard allows us to unify the code base of Azure PowerShell with minimal impact on users.
+With the Az module, Azure PowerShell is now compatible with PowerShell 5.1 on Windows and PowerShell Core 6.x
+and later on all supported platforms - including Windows, macOS, and Linux.
 
 Az is a new module, so the version has been reset to 1.0.0.
 
+## Why a new module?
+
+Major updates can be inconvenient, so it's important that we let you know why the decision was made to
+introduce a new set of modules, with new cmdlets, for interacting with Azure from PowerShell.
+
+The biggest and most important change is that PowerShell has been a cross-platform product since the
+introduction of [PowerShell Core 6.x](/powershell/scripting/overview), based on the .NET Standard library.
+We're committed to bringing Azure support to all platforms, which means that the Azure PowerShell modules
+needed to be updated to use .NET Standard and be compatible with PowerShell Core. Rather than taking the
+existing AzureRM module and introduce complex changes to add this support, the Az module was created.
+
+Creating a new module also gave our engineers the opportunity to make the design and naming of cmdlets
+and modules consistent. All modules now start with the `Az.` prefix and cmdlets all use the
+_Verb_-`Az`_Noun_ form. Previously, cmdlet names were not only longer, there were inconsistencies
+in cmdlet names.
+
+The number of modules was also reduced: Some modules which worked with the same services have been rolled
+together, and magement plane and data plane cmdlets are now contained all within single modules for their
+services. For those of you who manually manage dependencies and imports, this makes things much simpler.
+
+By making these important changes that required building a Azure PowerShell module, the team has
+committed to making it easier than ever, and on more platforms than previously possible, to use
+Azure with PowerShell cmdlets.
+
 ## Upgrade to Az
 
 It's recommended that all users upgrade to the new Az module. To do so:
@@ -31,10 +54,15 @@ It's recommended that all users upgrade to the new Az module. To do so:
 
 ## Migrate existing scripts to Az
 
-Major updates can be inconvenient. However, the Az module has a compatibility mode to
-help you use existing scripts while you work on updates to the new syntax. Use the
-`Enable-AzureRmAlias` cmdlet to enable the AzureRM compatibility mode. This cmdlet defines
-AzureRM cmdlet names as aliases for the new Az cmdlet names.
+The Az module has a compatibility mode to help you use existing scripts while you work on
+updates to the new syntax. Use the [Enable-AzureRmAlias](/powershell/module/az.accounts/enable-azurermalias)
+cmdlet to enable the AzureRM compatibility mode. This cmdlet defines AzureRM cmdlet names as aliases for
+the new Az cmdlet names.
+
+> [!IMPORTANT]
+> Even though the cmdlet names are aliased, there may still be new (or renamed) parameters for the
+> Az cmdlets that will be exposed. Don't expect enabling aliases to take care of the full migration
+> for you! After enabling them, test your scripts to make sure they don't need any immediate updates.
 
 The new cmdlet names have been designed to be easy to learn. Instead of using `AzureRm` or `Azure`
 in cmdlet names, use `Az`. For example, the old command `New-AzureRMVm` has become `New-AzVm`.