Deploying service providers together with old web sites

Perhaps it's better to open a new thread
for this argument.
If I have understood, to deploy a SourceID SSO
architecture in conjunction with some
previously deployed web sites (say WS1, WS2,
..), which perform user authentication,
I have to deploy a new site (IDP) and,
for each authenticating web site,
I have to deploy a SP (SP1, SP2, ...).
SP1 has to share the same http context
and the same session variables with WS1.
I also have to implement an AccountHandler
for each SP deployed. Each AccountHandler
implements an authentication mechanism in order
to access authentication information.
I leave unchanged the services offered by
previously deployed sites, even if
they are implemented in php or perl.
I only have to change the access to
authentication information.
Is it right?
I thank you very much.

Best regards,

Antonio Bonavita