Yeah, I suspect you could upgrade it, but might not be worth the hassle. If you do get it, read the terms closely but I suspect you can upgrade it. Also make sure the card really get you the benefits you want. Since you buy up, you probably don't need the checked bag benefit (I'm forget if C+ gets free bags but FC certainly does). Delta miles are really hard to use often. I was looking last night for D1 seats to Japan and it was 600k! My previous look for D1 to Argentina economy was 70k (booked like 8 month in advance which is great) but never lower than 510k for D1.

The AMEX Plat card (not the Delta one), might be good since it gets points which are very flexible and also you get into the Delta SkyClubs when you fly Delta, which are pretty nice these days. Or the Chase Sapphire Reserve would be good since you could use those points to book your C+/FC seats directly as well. Plus you get $300 back from the $450 you pay for any travel expenses.