This is how XMPP works in general. Each server is independent from the other, servers can communicate with each other. But an account on one server is not connected to an account on another server. These are completely different accounts, each with its own list of contacts.
You can convert the old XMPP account to Dummy to preserve history. Install the Dummy protocol, and create a new Dummy account. Open the database editor and find the section [Dummy_X]. This is the internal name of the account. If this is the only one Dummy account, then the name will be Dummy_1. Go to the contacts section in the database editor. Each contact has a Jabber_X subsection and Protocol subsection with "p" key. Change Jabber_X to Dummy_1 in both places.
Do it for every contact of the old XMPP account. After all the contacts of the old account have been converted to the dummy protocol (they now have a Dummy icon in the contact list instead of a Jabber icon), you can delete the old account from the account manager in Miranda and create a new XMPP account with a new server address.
Add and authorize your contacts again via new account than merge them with dummy contacts into metacontacts. Thus, each metacontact will contain two subcontacts: a dummy sub with the previous history from old acoount and a new contact in which a new history will accumulate as further communication
