Domain Trust Query

Hi,

I have a one way incoming non transitive trust to an external domain.
This trust was originally configured using server called say
MySrv2000. My domain at this time was pure 2000 domain running in 2000
native mode.

As part of a domain upgrade to 2003 I want to replace server MySrv2000
with a new server MySrv2003. The new server is setup "DCPromo'd".

Due to firewall restrictions to the external network DNS communication
is allowed only from IP 10.10.14.21 - this is the IP that is currently
assigned to MySrv2000. My plan was to move MySrv2000 to another IP and
move MySvr2003 to 10.10.14.21.

In testing, everything internal works fine after the move and DNS zone
transfers from the external network works fine to the new server.
Here's the problem; when clients attempt to access file shares on the
remote network they are unable to authenticate for some reason and are
denied access.

So my question is - What are the "rules" regarding domain trusts i.e.
If a trust is created with, in my case MySrv2000, does that server
have to remain as is or the trust is lost and has to be recreated. Is
the SID created with the trust, specific to the servers with which the
trusts were originally created?

Thanks

Paul