Downgrade AWS AMI instance to the new 'micro' AMI

AWS prices for micro AMIs clearly show the costs advantage to run a micro instance for a low-volume site.
Furthermore, you can build the entire scalable IT system on micros, and then just scale up the nodes that have to be scaled.

Unfortunately, migrating to the new micro AMIs it is not that simple. There is no simple way to convert your exiting small or medium AMIs to micros if these AMI were created a while ago and use S3 as their instance store.
You can easily convert your existing AMI to micro using AWS console if your AMI is
a) Linux based
b) EBS (and not 'instance-store'/S3) based. Since 'EBS-root' is quite a new AWS feature many existing AMIs are S3 based, so here the story starts.

If you got a S3-based instance the migration to micro AMI is not that simple.
Of course you can start a new micro instance and install there anything from scratch (it will be automatically created as an EBS instance).
If starting from the scratch is not an option, you can try to follow unofficial guides for converting an S3-based instance to an EBS instance. If the conversion is successful AWS console will allow to create a micro instance from your EBS-hosted AMI.

There are only three blog posts providing some help in S3 to EBS AMI conversion (I did not try any of them yet, none of them looks like an easy way to go):
http://www.elastician.com/2009/12/creating-ebs-backed-ami-from-s3-backed.html
http://www.full360.com/blogs/Migrating-Linux-S3-Based-AMI-EBS-Based-AMI
http://coderslike.us/2009/12/07/amazon-ec2-boot-from-ebs-and-ami-conversion/

I wonder why AWS  did not provide a 'ec2-' tools script and/or console support for these S3-to-EBS-root conversions.