This is a rather basic question below my overall skill level, so for that I apologize. A search proved fruitless, and I know that some of you guys will be able to give me a quick and thorough answer anyway. Please consider this scenario:

March: I am a Skymiles member, but because my previous status expired, I am back to being just a basic member.

April: Back up to Silver Medallion status. Purchased several tickets while Silver, some with companions, for use in September. Requested automatic upgrades where relevant.

May-August: Various activities cause my status to increase to Platinum Medallion.

September: Time for the April-purchased flights to take place.

Question: If I take no action after the original ticket purchase, will my September flights get upgrade attempts that reflect my Platinum status or will my Silver status at the time of booking be used? If the latter, what is the easiest way to re-submit/reconfirm so my higher tier kicks in for them? I believe that I've read that for some airlines (AA?), you need to resubmit the request or your new status won't be applied. Is this the case with Delta?

Thanks in advance!