From what I understand from a friend, pick an octane and stick with it. If you use the premium for a while then change, that's what can damage the fuel system. This is hearsay, so I don't know if it's true. It would totally explain a freak fuel line breakage in my parent's car, when I was a teen, lol.
The only difference between fuel types is how hot they burn, thus affecting performance. Lower octane fuel leads to knocking, head issues, etc. in newer vehicles so since you bought an SI you need at least the minimum factory recommended octane level.