LH Cargo is basically running a two-hub operation in FRA and CGN. As Daysleeper says, their biggest costumer is setting up shop in LEJ, making it an intercontinental super-hub. It stands to reason, should LH continue to provide the intercon uplift after DHL moves, that LHC will discontinue operating in CGN and move shop to LEJ. Either that or loose the contract, which is not presently something LHC can afford. One is of course hoping the Junk Adventure, sorry Joint Venture, will come to a grinding halt right quick, and that we'll get our own fleet of intercontintal widebodies. LH's idea of a partnership reminds me of a master and servant relationship - you can imagine which position LHC think they hold!
As for moving out of FRA in favour of LEJ ..... hmm, that would be rather difficult for them, for the reasons stated by ALLDAYDELI. Still, stranger things have happened in this business.
However, the fact that LEJ is in the middle of nowhere is one of it's main attractions for night operations.