ITs been done actully, go back in time to the S3 Virge lines ect, or older. My old Verge shipped with 2mb EDO and could take another 2mb of EDO if I wanted to buy it.
This isnt done today for a very simple reason, voltage ect.
DDR1, DDR2, GDDR2, GDDR3, GDDR4, and GDDR5 all run at diffrent voltages.
Also GDDR5 has twice the pins that the rest do.
Now add to that, depending on the card your looking at you could be dealing with 32bit or 64bit ram chips though 64bit is the norm today thats also there.
Another technical nightmare is timings, as these are hardcoded into the VGA bios, now you could say do what motherboards do, thats fine and dandy, but it still causes issues very fast when you start mixing video ram. Overall its a big hassle and way to much work and big chance of you instead fying your card.
IF you really wanna upgrade the ram though, feel free to.
Heat up the BGA ram till the sodder melts on the underside, and pull it off, or just scrape it off. Now clean the video card, get new BGA ram that meets the same timings, voltage ect and put it on there, you can do 256mb to 2gb if you want, just get the right stuff, and soder it on, and hope you sodered it on right.