If you're still within the 24 hour window you should be able to make changes for free as long as you find P availability. I just changed from an LH flight to an OS flight to try a different carrier and for the lie-flats. I actually received a refund of ~$6 due to, I assume, the difference in departure taxes between Germany and Austria. I wasn't able to get the routing I initially wanted, but that caused me to get creative and did build in time to check catching a game at Wrigley off my bucket list.