My son is joining us part way into our vacation in March. We have AP, but I bought my son a 3 day regular non-parkhopper ticket. On his first full day, we had already scheduled FPP at AK for the evening and luckily I was able to add him to our plans. (Except FoP, but I will try for a same day drop or let him use my FPP since I have ridden it a lot). We are now considering starting that first day at HS to try for a RotR BG but it may be a “game time decision”. If I haven’t added the parkhopper to his ticket yet, will his FPP for AK that night be safe if he taps into HS that am without a parkhopper option? I don’t really want to take the time to go to Guest Services to add the parkhopper in the am at HS, but I will if I have to.
Good question! Which is another way of saying I’m not sure. 
How did you buy his ticket? If you bought from Disney, I would call and upgrade it beforehand. If you bought it from a third party then to be honest I would upgrade it on the day or the night before; could you get to Disney Springs or a park then?
I would think it would be okay. The check is usually done retroactively in the middle of the night and I think it’s only looking to see if used FP’s had corresponding used admission.
Thank you for the replies. I bought the ticket from Disney. I hate to be so undecided and difficult (haha) but his flight doesn’t get in until 9pm and I’m unsure if we/he will feel like getting up early the next day to go to HS, especially if it changes to 8 or 7am opening. Maybe I’ll just risk it. I could take a screenshot of his AK FPP and if somehow they disappear before we add the PH, then I could see if they will add them back at AK. My husband, other son and I will have already been to HS a couple of mornings, so if the late arriving son isn’t keen on getting up early, then we will probably not do it anyway.