Transfer our company's data from one database to another.

We are estate agents in Spain. We have around 1,400 listings. Our database provider is called Milenio Plus and we wish to transfer all of our data from Milenio Plus to a new provider called Inmobalia CRM. Milenio provide us with an XML feed which allows us to export the property data that prospective buyers may see; thus we can export photos, descriptions and property details etc. . However we cannot export the 'private data' of any property such as the name and contact details of the owner, the viewing history (which cross refers to the contact database) and the supporting documents that we keep on digital file. We also cannot export our contacts and in particular prospective buyers and their history (what they have viewed) and property requirements. We also cannot export archived property details, that is to say property that has been on the market but has since been sold or taken off the market. All our correspondence is also stored digitally and cannot be exported on the xml feed. Not only does Milenio drive our website but if we were denied access to it then we would not have access to the information that we need to trade effectively so this is a very sensitive job that has to be done without reference to or assistance from Milenio.