My understanding is that an Architect always owns the copyright for his / her design drawings - a client pays for the right to use that design in the specific location it was commissioned for, ie. can't use that design on any other site without architect's permission and presumably paying a fee.
I think that as long as the client has paid in full for the work carried out by the original architect, eg. up to planning stage, then they are entitled to use that design, in the original location. If they hire another Architect to take over on the project, then the second architect should contact the first to ensure that they have been paid and that there are no issues outstanding, in accordance with the professional Code of Conduct.
Hope that helps - if not PM me.