It's not an $8K change fee. I will bet a good dinner in either Boston or Los Angeles (neither of which OP apprently will get to eat):

1. The change fee is $250.
2. The cost of the new routing is $A
3. The cost of the old routing was $B
4. A$-B$+$250 = $8,000 (add/collect)

Most likely reason is LAX-PEK in J is likely at full fare or close to.