Not stupid, friend, careful about unknowns :) and it's normal that a project wants to receive tested and validated pull requests - finding good testers is nearly as tough as finding devs I think.
And you're right: your situation is what docker usually excells at.
To address your concerns I suggest a three stage approach:
Pull/build from your repo in parallel with new volumes own port(s) etc. create some dummy data in there and rebuild with the test data, check that reconstruction works.
Then shut it down and remove it again. copy the volumes of your existing container and use that for the second instance.
If everything works fine there as well then shut down the old version, create a backup of the volumes and update it with your version.
This way you're safe and have an easy time seeing where and when something breaks at the same time.
You could also start with the last step and the backups only - but this way you can take w conscious step after the other. Your choice!