Because of the number of keying combinations, the keying cannot simply be changed UNLESS it is merely a case of rearranging the existing tumblers. It may be that you will need two of the same tumbler plates, but have only one now.
A full set of tumblers (four each of the five tumblers) is included with a new lock mechanism, but it's a bit spendy (~$80.00). Check the files in the TonneauLock folder of my library.
The part numbers for the tumblers are below, but I doubt they can be purchased separately (I'd give it a shot, though).
T1: 322261
T2: 322262
T3: 322263
T4: 322264
T5: 322265
