Changing Workspace AD from SimpleAD to AWS Managed AD

The direct answer is, no it's not possible to 'migrate' a WorkSpace from one Directory to another Directory. You will have to create a new WorkSpace for each user on the new Directory.

Depending on the customer, their use case and their deployment size I've got some suggestions here:-

For a handful of WorkSpaces you could look at cutting a custom "image". The default limit for the number of images a customer can have is 40 Images per Region. And then launch a new WorkSpace for the user based on the image. There are some things that wont follow the user though, but it's a good option for a small number: https://aws.amazon.com/premiumsupport/knowledge-center/create-image-workspaces/

For a larger number of WorkSpaces the best approach is to look at a 'user' migration where you can either self enable the user to migrate between their old and new WorkSpace (perhaps a Windows SMB file share on EC2 or even FSx) or a more orchestrated migration approach using Windows Roaming Profiles (sync to that file share) or a Profile Migration solution such as LiquidWare ProfileUnity:

https://docs.microsoft.com/en-us/windows-server/storage/folder-redirection/deploy-roaming-user-profiles https://www.liquidware.com/products/profileunity

I hope this helps you and your customer here!