Originally Posted by
findark
Not generally. It's possible the fare requires a UA-marketed flight but it's relatively uncommon. Otherwise the only difference between marketing carrier is sometimes inventory, but the conversion usually doesn't re-validate.
If you search Star fare listings on expert flyer, you'll see the fares(e.g. LAX-FRA) are generally the same(PNC2S), but LH may have the surcharge included in the fare listing while you may need to add these to the United fares. The fares are generally all the same cost.