OP here! Northern Ohio to Southern Alabama! We are very excited but nervous..
You are going WAY down South, bless your heart!
I think of "yankee" and "bless your heart" this way- nobody with good manners will say you are a Yankee to your face. That's just bad manners. But if you have an ugly attitude about being from the North, you might be called a Yankee behind your back. Same thing with "bless your heart". If you have good manners and you tell someone bless your heart to their face, it is meant as a sincere term of sympathy or commiseration. Now, well bred southerners have no problem talking behind your back, and it might mean something more snarky: "Can you believe Tiffani wore cutoffs and flip flops to church? Bless her heart, she looked like a bum!"
If you want iced tea, you can get it any day of the year. But you WILL get sweet tea unless you specifically ask for Unsweet.
Strangers will strike up conversations with you everywhere you go. Everyone waves to each other.
You will be asked where you go to church. If you don't go or haven't found a church yet, you will be invited repeatedly to go to whichever church the speaker goes to.
Adults are called ma'am, sir, or by Mr/Miss First Name. Children do not call adults Mr/Mrs/Miss Last Name (except teachers).
Southern Alabama is really into Mardi Gras, as much as New Orleans. It is a really fun time of year.
You will start to use "southernisms" like y'all, fixin' to, and buggy (instead of shopping cart).
People will tell you to have a "blessed day". You will be wished "Merry Christmas", not "Happy holidays". People are very upfront about their religion (in the South, usually Baptist, Methodist or Church of God or some other Pentecostal denomination). If you aren't religious, be prepared to have people try to talk you into going to church.
Football (college, SEC in particular) is big in Alabama. Huge. Massive. You will be expected to choose either Alabama or Auburn as your team. Friendships might be affected by your choice. Did I mention football is big in Alabama? I cannot overstate this.
Most of Alabama is still very Old South compared to the "New South" of Atlanta, Nashville, Dallas, or Charlotte. Not many folks leave the little towns they were born in. Not many people from somewhere else settle there. People are very nice, but you might encounter the "old boys' network" that makes it hard to break into the country club, or get a promotion, etc.
Kids will be expected to attend manners and deportment classes around middle school age. This will probably be called Cotillion and it is a Big Thing for the middle and upper middle classes. If your kids don't participate, they might not be considered for cheerleading, student government, or other "popularity" based activities in high school.
Overall, living Down South is great, but there are some things that will seem strange- and there are some things that ARE strange. You will come to love most of it, and learn to live with other things.