I used to do this regularly when I was living in Europe and travelling back and forth to the US. The rules can be a little tricky and I assume that this is still the case. For example, I may have booked AMS-IAH-AMS.... flown the AMS-IAH leg but decided to use points to fly back to AMS in LH F. Sometimes I was forced to use the exact same routing when I was rebooking, meaning IAH-AMS. It wasnt an issue for me as I was back and forth once a month, on average, between business and personal.

Just make sure you understand the rules. I was not able to use the credit freely like you would normally assume.