SD - 3.16.3.0

I have updated the definition for an aircraft and then open a previously saved route that uses this aircraft. SD tells me that the definition has changed and it will use the one in the library. So far so good. I then make some changes to the route and re-save it. However, when I re-open it again in the future, without modifying the aircraft definition, it again tells me that aircraft definition has changed.
Q. How can you get a route to recognise and adopt the aircraft definition that is in the library so that you don't get the warning message each time you open the route?