You can select on file open dialog (stu or sta file) to which unity version you want to convert original file.
So you need first open file on newer v13 and on file open select v12 (or older v11.1) version for downgrade, then rebuild project and save to archive format (*.sta).
You also maybe need select firmware version to lower on hardware config so that file open won't go to file converting error from sta-file.
(If you have newer firmware selected on v13 version than v12 installation supports/knows)
Client needs then donwload v12 version to CPU, otherwise online connection isn't possible because project build version isn't same anymore on PC than on CPU.
Depending of project settings of old project all data is lost or maintained on CPU at download