How many legitimate cases when Origin AS in BGP announcement changed by another AS?

Tue Jan 4 16:05:10 UTC 2011

On Tue, Jan 04, 2011 at 04:38:19AM -0800,
 Akmal Shahbaz <akmal_shahbaz at yahoo.com> wrote 
 a message of 443 lines which said:

> I am looking for example routing policies when any AS receiving BGP
> advertisement changes Origin AS in BGP AS set attribute to remove
> the received AS number and puts its own AS number.[legitimate cases]

When the old origin AS was a private one? Today, with 32bits AS,
everyone should have a public AS number but, in the real world, some
people do BGP with private AS that the upstream provider has to
change.