How I do it:
Open the projects in construct 2.
Copy paste the objects first to be copied from the old project to the new project first.
Then create an empty event sheet in the target project, and copy paste the content of the event sheet from the old to the new project.
I do this for all the event sheets.
You could also try and work your way through with some svn like approaches .. but I'll bet you will be tinkering longer with it to get it working then the previous mentioned manner.