A lot of the GPUs have fixed function hardware to accelerate parts of encode/decode. If the new codec is compatible, sure.
But often a new codec requires decoders to know how to work with new things that the fixed function hardware likely can't do.
Encoding might actually be different. If your encoder hardware can only do fixed block sizes, and can only detect some types of motion, a driver change might be able to package it up as the new codec. Probably not a lot of benefit, other than ticking a box... but might be useful sometimes. Especially if you say offload motion detection, but the new codec needs different arithmetic encoding, you'd need to use cpu (or general purpose gpu) to do the arithmetic encoding and presumably get a size saving over the old codec.
But often a new codec requires decoders to know how to work with new things that the fixed function hardware likely can't do.
Encoding might actually be different. If your encoder hardware can only do fixed block sizes, and can only detect some types of motion, a driver change might be able to package it up as the new codec. Probably not a lot of benefit, other than ticking a box... but might be useful sometimes. Especially if you say offload motion detection, but the new codec needs different arithmetic encoding, you'd need to use cpu (or general purpose gpu) to do the arithmetic encoding and presumably get a size saving over the old codec.