OK thank you to @MadSystem for pointing me in the right direction. Checking out this thread: Association .uproject / unreal are broken - Debugging, Optimization, & Profiling - Epic Developer Community Forums
As suggested in the bottom of the thread I manually copied the ‘UnrealVersionSelector.exe’ from ‘C:\Program Files (x86)\Epic Games\Launcher\Engine\Binaries\Win64’ to the engine binaries folder: C:\Program Files\Epic Games\UE_4.15\Engine\Binaries\Win64’
Then set the uproject file to associate with the newly placed ‘UnrealVersionSelector’. The right-click menu came back and I can finally rebuild Visual Studio project files. Yay!
Thanks!