Use your backup copies is the only way you'll get to open them back in 2017. You'll just have to keep moving forward in 2019 for these files unless you made backups before trying them in 2019
To Restore Previous Version SW2019 must be available. In your case it's not.
Upload your files and ask forum if somebody with SW2019 would be able to Restore Previous Version or convert files into Parasolid (most friendly to SW format).
Then open files in SW2017 and use FeatureWorks->Feature Recognize procedure to recover original files.