If the Runtime Mesh Component is installed in the 4.14 Binary Engine, upgrading a copy of a code project from 4.14 to 4.15, even if it does not use that plugin, will result in the upgrade process hanging indefinitely when generating the project's code. This requires that the UE4Editor.exe and UnrealHeaderTool processes be killed in the Task Manager window.
The project is copied successfully before the upgrade, so after killing the above processes, it is possible to generate the project code and build the project. Also, making sure the Runtime Mesh Component plugin is installed in 4.15 before upgrading the project allows the project to upgrade without any problems.
Make sure the Runtime Mesh Component plugin is installed in 4.14, and not installed in 4.15.
The upgrade process will get stuck at Generating Code, and the UE4Editor.exe process starts to show a very slow but steady increase in Memory use.
The upgrade process completes, perhaps with a warning that the Runtime Mesh Component plugin is not installed.
Make sure the Runtime Mesh Component plugin is installed in the 4.15 version of the Engine before upgrading the project.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44075 in the post.