From someone that has worked for Asurion, it all depends what is available at the time. Which honestly, shouldn't be so hard to understand but getting reemed out by mad customers for a year and a half, people just don't understand. All those warranty phones sent in, if possible get cleaned, fixed, and tested. When Asurion gets a shipment in and thats the phone you are replacing that will be the phone you get. Even if Asurion was out for 6 months they can still get a shipment of that phone in. Yes after the phone is discontinued it is more unlikely that the phone will come back in stock. Though I have seen it more times than once when a phone they didn't have in stock for months all of a sudden comes available during order process.
Not only this, but if there is a limited amount of a phone left, the system automatically chooses a comparable phone that is more abundant. Basically, that keeps the system from over ordering a phone causing backorders and such.
Keep in mind that just cause a certain phone is the successor of the phone your replacing doesn't mean that will be the replacement phone. Always loved how people EXPECT the upgrade to their device if theirs isn't available. Since it has to be a comparable device, if something is closer in specs that will be the replacement, not the upgraded device.
There is a lot to Asurion's insurance replacement process but it is all about reading the Insurance Coverage Terms. There is a lot in there that people that don't read and then get mad at the CSR's for them not reading the Terms. My favorite is when a parent punishes their child by not replacing their phone for like 4 months and when they do call in and find out that they only had 60 days to call the claim in they get pretty upset. In the end the parent f***ed themselves.