Avoid the use of upcasters when possible. XStream allows aliases to be used for fields, when they have changed name. Imagine revision 0 of an event, that used a field called "clientId". The business prefers the term "customer", so revision 1 was created with a field called "customerId". This can be configured completely in XStream, using field aliases. You need to configure two aliases, in the following order: alias "customerId" to "clientId" and then alias "customerId" to "customerId". This will tell XStream that if it encounters a field called "customerId", it will call the corresponding XML element "customerId" (the second alias overrides the first). But if XStream encounters an XML element called "clientId", it is a known alias and will be resolved to field name "customerId". Check out the XStream documentation for more information.