you can download free driver updates from your gfx card manufacturer's website. updated drivers, in most cases, can improve gfx card performance. you can also download gfx card bios updates which most of the time improves performance and fixes problems.
but you cannot upgrade your gfx card hardware online. if you have to upgrade your gfx horsepower, you might have to buy a more powerful gfx card.
Every now and again a certain GPU will be unlockable to a more powerful version, some 6950's can unlock to a 6970 i believe, and the oldschool 9500 pro would frequently unlock to a 9700 pro, and as for it being legal of course it is, you own the hardware after purchase. and as long as its only a softmod aka firmware update then your warranty is still good, however if you have to hardmod it then you void the waranty.
Also sometimes when you RMA your faulty card they can send you new hardware. It happened to me tree times. I send 7800GT and got 7900GS and then that one got bad RMAed it and got 8800GS after a year or so RMAed it and got ATI HD6670 which I is about the same as 8800GS but more power efficent...that came in handy since I wanted to build media center...instead I bought GTX285...and I had nothing but a trouble with it RMAed it 3 times but always got GTX285 back...was hopping that they would send 560Ti but no I gave up and bought 6950 flashed it to 6970 was very happy so I got 3 more over one year span lol