Can headlight domes (not bulbs) be replaced?

Yes, they can, if by the domes you mean the headlight housings. It doesn't take long if you've done it before, maybe half an hour. A bit longer if you need to figure it out.

Have you thought about restoring them?

 

As an example

https://www.ebay.com/itm/371008625885

 

It depends on how bad they are. If they are really yellowed, you will need to sand the top layer off with varying grits of sandpaper (I prefer wet and dry) from 1500 to 3000. If not so bad, you might just be able to use some cutting compound applied with a machine buffer. This is then followed by machine polishing to restore the clarity of the lens. You can do it all by hand, you just expend more energy and it takes longer. There are plenty of YouTube videos, but watch out for the hack jobs (toothpaste, etc) – you can skip them. There is only one way to do the job and that is to do it properly.

How long it lasts will depend on what UV protection top coat you apply. If you don't put any UV protection on top, you will need to polish them every 3-6 months with something like Turtle Wax headlight restorer to keep them clear.

 

Try this first. Less than an hour’s labor and $14
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