A project created in 2019 Classic can be opened in 2017 and most things should work. However, there were some changes and going forward Adobe could apply an update to 2019 Classic that breaks compatibility. Even Adobe cannot guarantee that will not happen. It's not a scenario that I would risk.
If you are referring the new UI 2019 version that also ships, there is no compatibility. The code is very different.
It could be worth contacting Customer Services to see if they will sell you another copy of 2017.