The WCF Plain Services in the OpenAccess SDK are generated with the Add OpenAccess Service wizard using the default T4 templates. The definition of the method you mention - SaveRentalOrders is added manually in the generated interface.
The reason for the exception is that when the interface is overridden, the implementation of the SaveRentalOrders method remains in a partial class which is not replaced during the code re-generation. A solution for this scenario is to add the definition of the SaveRentalOrders in the interface.
In the C# version of the sample we use partial interfaces to add the custom business logic, but this scenario cannot be achieved in VB.NET because of the specifics of the language.
To separate the custom business logic from the generated code, you can create custom interfaces that inherit the auto-generated interfaces to extend the logic. In that way after "Updating" the service, no additional configuration will be needed and no information will be lost. But bear in mind that for this scenario the web.config file of the service project needs to be configured appropriately - let us know if you would like further details on that.
Basically the scenario for which the Add OpenAccess Service wizard is used is to add services, and then since there is no automated code generation apart from the wizard itself, you are free to make changes and extend your service in any possible way. In case you would rather prefer an automated update logic, we will appreciate if you could specify a use case for this scenario and what are your expectations for the end result of the "Update" process, so that we can enter it into our backlog.
I am looking forward to hearing from you.
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.