Downgrading your firmware is fine and as safe as anyother stock flash, with one exception. For Samsung devices obsessionally employ rollback protection across certain releases, primarily if there is a security issue that gets fixed in later firmware.
To determine if the anti rollback counter will be incremented f next version. Look at the 5th character from the end of both your current firmware and the one you want to flash. You will find a number there, which is the bl version (or anti rollback number) for said firmware.
For example: Firmware SM-G950USQU2BRB1. Count back 5 from the end and you get 2. This firmware will work on v2 & v1 firms and will also upgrade V1 firms to v2.
sorry I didn't get what would happens
btw now I have this
and id like to downgrade to
also id like to know where to find the rom as mentioned; I saw that on sammobile there are many and I don't know which one to chose. I found just the samsung s9+ version