It must totally depend on the company and probably even the adjuster you are dealing with. When we were hit, I had Geico and the person who hit me had Nationwide. They pulled the "not talking to insurance" BS so even though we had a police report noting her at fault, they wouldn't pay out until they spoke with her supposedly. I talked to Geico about the process of my paying our deductible and going that route for our seat as well. Geico claimed to have a person "certified to inspect and determine if the seat was safe to use". HA. I told them no such person exists and she kind of back peddled out of it and we never got into enough detail to see what their true policy is.
I just hounded Nationwide every single day until I finally mentioned that I have a sick child, and if I had to replace my seat out of pocket in order to get her to her next doctors appointments over 6 hours away, he could be sure that they would be paying for more than just straight damages and a car seat. They miraculously got in touch with the lady who hit me that very day. After a month. Imagine that. ANYWAY, to make a short story long, we did it all through email. I emailed the page in Graco's book that said replace the seat after any accident as well as pictures of the seat. He did try to lean toward prorating, but I make a very good argument. Ahem. So he just went online and found the cheapest retail price and cut us a check for that.
If it had been a discontinued seat, I would imagine they would have done the same thing if still available but at a discounted price.