Hi @buzzears this is the info page for Samsung Galaxy S9 https://doc.e.foundation/devices/starlte, the supported models are listed at the foot of the page. To avoid surprises you can boot the phone into Recovery mode [1], and you should also see the devicecode starlte in fairly small writing.
As you may not be certain of the device history you could boot the phone into Download mode [1], take a photo of the screen and post it here, we can tell you if it gives any negative clues!
The Easy Installer should leave the phone untouched if it were “wrong” in some way, but best to read, ask and prepare!
With the thought of an escape route to regular OEM Android, you can search for a Factory ROM [1], and read about how to flash the device back to that. Be aware that if you do flash a Custom ROM, then revert it to “Original” Samsung, a trace will remain on the phone, arguably spoiling your chance to return it according to those terms!
[1] Know your hardware - Samsung
You will find Easy Installer installs Andoid 8 Oreo on an S9… [LIST] Devices working with the Easy Installer
Good luck