If premium is only suggested, but not required, and regular is ok to use according to the owner’s manual, then using regular will be ok. The engine computer will soon realize regular is being used b/c the engine will start to ping, and the engine computer will respond to that by retarding the ignition timing and perhaps other things, which will end the pinging. The downside is the acceleration might be a little slower than before. If you want to worry about something, worry about the ping sensor. It is critical that is working correctly for this all to work. When pinging occurs, it can damage the engine. In a big way, like burning a hole through the piston. Yeah, like you got a welder inside your engine, it gets that hot and bothered when it pings. I expect though you’d be fine to use regular, and no damage was done to the engine using regular in the past. Bad pinging is usually very obvious to the driver, it sounds like someone is rattling a coffee can full of nuts and bolts.