To convert a prospect in CRM into Sold-To-Party, set a validity date for the roles...
Here is an example:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:glob="http://sap.com/xi/CRM/Global2">
<soapenv:Header/>
<soapenv:Body>
<glob:CustomerCRMChangeRequest_sync>
<MessageHeader></MessageHeader>
<BusinessPartner>
<InternalID>110000061</InternalID>
<Role actionCode="02">
<RoleCode>BUP002</RoleCode>
<ValidityPeriod>
<StartDate>0001-01-01</StartDate>
<EndDate>2014-11-18</EndDate>
</ValidityPeriod>
</Role>
<Role actionCode="01">
<RoleCode>CRM000</RoleCode>
<ValidityPeriod>
<StartDate>2014-11-18</StartDate>
<EndDate>9999-11-18</EndDate>
</ValidityPeriod>
</Role>
</BusinessPartner>
</glob:CustomerCRMChangeRequest_sync>
</soapenv:Body>
</soapenv:Envelope>