In order to flash the BIOS, you need a CPU. Sometimes newer batches of boards come with newer BIOSes so see if it isn't written "Sempron ready" on the box of the board.
At worst, install the Sempron CPU & flash the BIOS. Before you do so, the Sempron will be recognised as an AthlonXP. The newer BIOS versions just add the proper CPU identification string, it doesn't do much else. Your board supports the Sempron's 166mhz FSB without a problem, you just need to make sure that you have at least PC2700 Ram. & don't get a 3100+ Sempron as it will not work on a Socket-A motherboard.