The slower speed might or might not be a problem, it depends on the graphics card's timing circuitry. In my experience it's almost never a problem in this kind of applications, would guess the designers left margins to be able to use whatever pin compatible chip that happened to be the cheapest at the moment.
However, you'd also have to do some magic to sort the pin 1 and 27 differences between those chips.
Pin 1 is VPP on the 27c256, A14 on the 28c256
Pin 27 is A14 on the 27c256, /WE on the 28c256
Basically you need to:
1) Break the connection between PCB socket and chip pin 1. Vpp on the 27C256 is the programming voltage supply, only used during programming, and is probably tied to ground on the PCB.
2) Wire PCB pin 27 to chip pin 1, to route A14 to the correct pin.
3) Bridge pin 27 and 28 on the chip with a suitable resistor (let's say 10 kOhm), to tie /WE high. While you might get away with leaving /WE floating, it's good practice to tie it high.
4) Break the connection between PCB socket and chip pin 27, to avoid tying the A14 line high with the above step
The easiest way to achieve that would probably be to:
1) Fold up chip pin 1 and 27 before inserting it in the socket.
2) Solder a wire to chip pin 1, strip and tin the other end of the wire, and insert it in PCB socket hole 27.
3) Bridge pin 27 and 28 on the chip with the resistor.
If you decide to do the above, please double and triple check it first. Tired as hell tonight, won't give any guarantees I got it right 😀
Otherwise, an alternative would be to get an UV erasable 27c256 chip instead, and keep your fingers crossed it arrives erased and ready for programming. If not, you can get a cheap (~€10) UV hand sanitizer lamp and erase it. Would not gamble on buying old 27 series OTP EPROMs on Ebay etc, chances are they're already programmed.
