I found a very reasonable flight from MEM to HKG that goes through NRT for only $977. Most flights that go to NRT now from MEM are around $2000 these days. So, I was wondering if I took the MEM to HKG and HKG to MEM round-trip flight, then I could change my baggage final destination to NRT instead of HKG and leave the airport when I get to NRT. Once I leave Japan, I could check-in at NRT and then just use MEM as my final destination. Is that a successful itinerary? What issues would there be? This is on Delta.
Generally you cannot do this. Your baggage will be checked to your final destination. If you ask them to check it to the midway point, they will know what you are doing, and either refuse, or cancel your ticket entirely.
The only two exceptions I'm aware of are:
If you have a long, overnight layover, the airline will often allow you to retrieve and re-check your luggage so that you can take it to the hotel with you.
If you have to clear customs at your layover, you must retrieve your luggage to clear customs.
This is the case next week for my girlfriend and I, going from Guatemala to Houston, Houston to Florida--we'll hop off in Houston, and throw away the Florida leg, but as Houston is our port of entry, we will be required to retrieve our luggage to go through customs.
Also note that this will only work for one-way flights! Once you skip any single leg of an itinerary, the rest will be cancelled. So if you're trying to buy a round-trip ticket for MEM->NRT->HKG, then HKG->NRT->MEM, as soon as you skip the NRT->HKG leg, the return trip will be cancelled.
Put simply, no, you can't do this - for multiple reasons.
Firstly lets ignore check luggage for a moment.
Any time you fail to board a flight, the airline will almost certainly cancel any subsequent flights on that ticket. So if you book MEM-NRT-HKG-NRT-MEM, then fail to board the NRT-HKG flight, then the remainder of your ticket - including the NRT-MEM segment - will be canceled and you'll be stuck in NRT. Thus even if you don't check bags what you're suggesting isn't going to work.
The second problem is going to be your checked luggage.
What you're referring to is "short-checking" of luggage, and we already have an answer on that topic over here, but in short most airlines now days will NOT let you do it as they presume you're planning not to take the connecting flight. This is particularly true for international flights, where you'll need to go through immigration to collect your bags - and the airline generally doesn't want you doing that on a flight connection due to the time it takes.