Welcome to FT, and congratulations on your upcoming status change!
Based on a recent thread on the same topic (
http://www.flyertalk.com/forum/unite...st-flight.html), checking in with LH and then attempting to select a seat on UA for the next flight may cause problems.
If you are not worried about checking in with LH at exactly T-24, then you will probably have better luck waiting until T-24 of your FRA-SFO flight. At that time, use UA.com ("view my reservation", not OLCI) to select an E+ seat for FRA-SFO. After that is done, then proceed to check in with LH for PRG-FRA.