I currently use a Nikon D-600 and my wife has a Nikon D-7100 with dedicated lenses.
Both cameras are virtually identical in size, control interface placement and menu set-up. They share the same batteries, remote release and SD cards. We can swap with minimal confusion.
Where I get vapor-lock is when a guy hands me his CoolPix camera and asks me to take that once in a lifetime shot of him and his wife with Elvis on their bucket-list vacation.
When I upgrade, it will be to a D-810 with which I expect a reasonable learning curve but I will not be starting out re-inventing the wheel and will be able to continue the adventure.