Don't use looked up values. If your relationship is set up with inequality operators to match on a range of values, you can put the actual field from the score-to-letter table on your layout listing names and grades.
Here's a second option that can be done manually or via script everytime you update the lookup table:
Show All records
Put the cursor in the score field
Select Relookup from the Records menu.
No re-importing of records needed in either case...
Many thanks Phil! both methods work great, saved me a lot of time!