Can I change an adult brief in a lift chair?

Can she stay in her care facility instead? This is more than you might want to take on at home. Changing her brief would be fairly difficult no matter how you choose to do it. You don't mention if she has dementia, but that and other reasons could make it difficult for her to transfer to commode or toilet. Just remembering to do so is hard for dementia patients. If she doesn't have dementia, there's still a problem because her limitations might make it impossible for her to hold it as she transfers or walks with walker. Those aren't simple tasks for weaker, older people. She may need help each time. Are you prepared to do that as well as wipe her, clean up after her when she misses the target, etc? You'll surely need at least a hospital bed, but I wouldn't bring her home if I were you. There are odors involved and extra laundry. Also your sanity.

If the chair can recline, & Mom can roll to the side well, maybe. If the chair does not raise to your waist height you will break your back in no time.
May need a hopsital bed for yourself.

If she can no longer get up to use her walker or even transfer onto a commode then I encourage you to reconsider getting a hospital bed - it would be so much easier to change her linens and clothes, to deal with toileting, and to reposition her so she doesn't end up with pressure sores.

I am in agreement with "funkygrandma59"
But to answer your question you "can" change a brief in a lift chair. (In theory you can change one almost anywhere)
but the sections of the chair, the back then a space then the seat will make it more difficult.
If you have to change the brief in the lift chair place a solid piece of non porous material over the chair. This will close the gaps in the chair and prevent any leakage from running between the sections of the chair. Place a disposable pad on top of the material before she sits down.
Lay the chair back as flat as possible then change the brief just as you would if she were in bed. The disposable pad can be removed with the brief and you can scoot another one under her.
If she is getting to the point where she is not mobile then a Hoyer Lift would be of more use.

Your post is a bit confusing to me. You first ask if you can change a diaper in a lift chair, but then go on to say that she will transfer to bedside commode near her.
Her diaper will be much easier to change when she is on the bedside commode than it will be trying to do it when she is sitting down in a chair. It will be easier in the chair if you recline it pretty far back(in sleep position)before trying to change her in the chair,