Child to Adult No Expiration Ticket & FPP (I anticipate a problem)

My son has a child's no-expiration ticket and has used one day of it. His next trip is likely going to be a week after he turns 10. So I'm worried about reserving FPP because I'll be reserving for days where he is a Disney adult (10yo) and his linked ticket is a child's ticket (3-9yo). I'm worried that (a) I won't be able to make FPP reservations at all for him, or (b) they'll disappear on his birthday. I will probably just go ahead and buy a separate (adult) ticket for the next trip, and then swap his no-expiration ticket on that trip to an adult one, just to be safe. But I thought I'd ask -- does anyone have any experience with this?

I don't have the experience of making FPP but I can yell you it was a quick and easy process to update the ticket from child to adult at GS.

It's very easy to swap a non exp child's ticket for an adult one. I did it at GS without a problem. Not sure about booking FPP but can't imagine it being a problem as long as he has a ticket linked. Maybe call Disney and check about the FPP? Seems a shame to buy a separate ticket if you have all those non exp. days left

We had a child ticket for my son, but he was 10, so an adult in Disney's eyes. We linked to our magic bands with no problem and made FP+ with no problem, if I recall correctly.

@Armadillo_Alert, @jvandcs, @Agent_C -- thank you for your responses! I'm probably over thinking the FPP issue. We'll give it a try (and save a few $$s in the process).