I agree. It is a pain to replace.
I had a RecordStore that had 5 records in it, and I wanted to replace those, what I ended up doing was to clear out the RecordStore, then put in my new values. This worked fine for me.
I think if you wanted to find a particular record and replace just that one is a bit cumbersome in J2ME. This is because of how you iterate through the RecordStore, because getNextID or getNextRecord automatically move you up in the RecordStore, so you can't do both at the same time. Meaning if you get the next record to see if it is the one you want to replace, then you can't get that record number. You can go next and then previous to get that record number, but that is poor design.
I know that has to be a better way, and I think I had had one before, but can't remember it.
Good Luck
Mark