We have registered version of JMS 1.2 installed in Joomla1.5.20

Our URl Master Site URL was usentrepreneurtoday.com/

for which we had different slave sites like

newyork.usentrepreneurtoday.com/

pennsylvania.usentrepreneurtoday.com/


Now we have moved the master site to americanentrepreneurship.com

and the slave sites are also been created as

newyork.americanentrepreneurship.com/

pennsylvania.americanentrepreneurship.com/


All are working fine except the issue we are facing is when we try to change the external URL, given in the menu it changes but after some time the value is been revoked to the old url which was already there.

For example in the attached image we have changed the home url "newyork.usentrepreneurtoday.com/" to "newyork.americanentrepreneurship.com/", it works fine but it gets changed back to "newyork.usentrepreneurtoday.com/" after some time.

As we changed the root domain from "usentrepreneurtoday.com/" to "americanentrepreneurship.com/", do we need to register the new domain with JMS

You can register JMS on the new server but JMS does not uses that in any manner to compute the link to the images.

What you need to verify is that the slave site definition JMS use the correct domain name.
When a domain is NOT present in the list of slave site, this is the master website that is displayed.

So verify the list of slave site definition and ensure that when you open / resave the slave site that you have a success message (blue message).
JMS has a "master index" with the list of the domains that is updated when any slave site is sucessfully saved.

If you have a doubt concerning the "images" path, you can also use a browser debugger like firebug or other to verify the path used by the images.

You can also verify the page source to verify the images path and perhaps discover somewhere a wrong domain name recorded.