sure, you could do that, but i think you should wait until you get to beijing because buying tix is as easy as: 1) making a phone call and leaving cash with your concierge; or 2) walking into any travel agency.

although less and less true, agents at the origin city often have access to more discounted inventory than agents in other cities. in china, discounts seem to bottom out at around the 3 day window (there are exceptions to this rule, of course).

also, have you considered flying directly from pek to sgn. i know there are flights, and i would guess that the cost is no more than $250. so, once you back out the cost of your pek-sha leg, you're looking at a ~$150 penalty for ditching your pvg-sgn ticket, assuming you can't refund/change it.