The question below has an incorrect answer being marked as corrected.
Following advice from your consultant, you have configured your VPC to use Dedicated hosting tenancy. A subsequent change to your application has rendered the performance gains from dedicated tenancy superfluous, and you would now like to recoup some of these greater costs. How do you revert to Default hosting tenancy?
Suggested correct answer: Use the AWS CLI to modify the Instance Placement attribute of each instance and the VPC tenancy attribute of the VPC
Actual correct answer: Create AMIs of all your instances. Create a new VPC with Default as the hosting tenancy attribute, and use them to create new instances using Default tenancy.
Explanation: Once a VPC is set to Dedicated hosting, it is not possible to change the VPC or the instances to Default hosting. You must re-create the VPC. Further information: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/dedicated-instance.html#change-tenancy-vpc
II believe the chosen answer is correct
Please read this from the link you provided In your post
"You can change the instance tenancy of a VPC from dedicated to default after you create it. You cannot change the instance tenancy of a VPC to dedicated"
So it is possible to change to default which what is being asked in the question, but not possible the other way around
Looks like the explanation for that question answer is a little bit off – as Ahmad Alhady rightfully pointed out, according to the link posted in the explanation it IS possible to change the instance and VPC tenancy for already deployed resources, as long as you are going from dedicated -> default and not the other way around.
The link and correct answer reflect this – I’ll get the explanation updated to do so as well.
Note that this functionality was introduced in late 2017 https://aws.amazon.com/about-aws/whats-new/2017/10/switch-the-tenancy-of-your-vpc-from-dedicated-to-default-instantly/ – previously it would not have been possible to do this!
You can change the instance tenancy attribute of a VPC from dedicated to default. Modifying the instance tenancy of the VPC does not affect the tenancy of any existing instances in the VPC. The next time you launch an instance in the VPC, it has a tenancy of default, unless you specify otherwise during launch. You can modify the instance tenancy attribute of a VPC using the AWS CLI, an AWS SDK, or the Amazon EC2 API only. Reference: Change the tenancy of a VPC.