When you migrate 2016 projects during the 2018 install process, the files are copied to the 2018 installation. Your 2016 installation remains completely intact and independent of the 2018 installation - the only change being that the folder is renamed. You can still open / work on / save your 2016 projects in the 2016 program. At the same time, you can open / work on / save 2018 projects (including those 2016 projects which were copied to the 2018 folder during the migration process) in the 2018 program. Hovering over a Project on the SoftPlan Start Page will show you the full path to the project files, so there shouldn't be any confusion as to where the project is located / what version it is.
It is strongly recommended to always open any / all projects from within the SoftPlan program in which they are to be saved. Remember that double clicking on a file will open it in the most recent install of the SoftPlan program, regardless of what version it was last saved in or its location. This is a limitation of the Windows operating system and not something we have any control over.
If you should accidentally open a project in the wrong (newer) version, DO NOT SAVE. Close the program and then delete the project (.spp) file. Open the project in the previous version, the project file will be recreated and you will be able to access your drawing(s). However, once drawings have been saved in a later version there is no going back.