Update
The required steps are as follows (All from powershell)
1: Login
Add-AzureAccount
2: Switch to azure resource mode
Switch-AzureMode AzureResourceManager
3: Create dictionary to represent the new farm
$whp=@{"serverfarm" = "<newfarm>";}
4: Switch Farm
Set-AzureResource -name <sitename> -ResourceGroupName <groupname> -ResourceType Microsoft.Web/sites -apiversion 2014-04-01 -PropertyObject $whp
You can get a list of resources by executing Get-AzureResourceGroup
once you are in resource mode
Update 2 10/29/2014
You can now do this from the preview portal
1: Login to the preview portal (portal.azure.com)
2: Pull up the card for the website you want to change
3: Click the ellipsis on the action bar near the top
4: Click Web Hosting Plan
5: Profit