The problem is that you spend an extra $200 for that upgrade from Apple.
Doing what you propose, you spend $200 + $40 = $240 for 16GB. If you upgrade to 16GB and discard the the original RAM, you pay $0 + $80 = $80 for 20GB of total RAM, of which you could use only 16GB. It gives you 4GB to sell on eBay if you have the time or inclination to get a small portion of that $80 back.
/Jim
that's it you see, i didn't have the time or inclination to do what you propose. and i knew that the 2 gig dims would just rattle around my house 'til they got lost or broken. this way no dims were hurt during the making of my iMac.