How to change Account, Opportunity owner and Sales Rep on Leads

Hello, we are looking a building an integration that will update the Owner on Accounts, Contacts, Opportunities and the Sales rep on Leads. Before our developers begin we need to determine what fields are available through web services for owner ship changes and what user data needs to be mapped to them to provide the ability to change Ownership Accounts, Contacts, Opportunities and the Sales Rep on leads. I have reviewed the WebServices guide but did not find any clear cut information, on the change of Ownership. I have been told that the user alias can be used, but was