I was in your same situation about a year ago.

After thinking it through, I decided that in order to gain one extra cog on the back and move the Sora button downwards a few inches, I'd have to pay around $200-$300.

Obviously, I didn't do it.

My solution?

I sold my close-to-new Sora bike for $675 (with an upgraded wheelset). I bought a new bike with a full Campagnolo Veloce drivetrain for $700.

Solved.