[CentOS] Converting from RHEL to CentOS questions.

I have servers that are running:

Red Hat Enterprise Linux ES release 3 (Taroon Update 5)
and
Red Hat Linux Advanced Server release 2.1AS (Pensacola) [which I think RH
renamed as Red Hat Enterprise Linux AS release 2.1].

The servers are running fine. Is it possible (and if so, how) to convert
these to the equivalent releases of CentOS, without doing a complete upgrade
(I want to mess with the systems as little as possible)? I work at a small
company and we simply can't afford to pay for the renewal costs to be able
to keep them updated from Red Hat.

--
-ste
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