It is possible to remove the freewheel without killing it. Although you do end up with a rather large pile of ball bearings, all trying to run away and be free!.
There is no way to convert the stock hub to a cassette type freewheel. That requires a different Bafang hub. Some of the new Sondors have this upgraded hub. I think it is a 500 W motor. You need a shimano type spline on the motor hub. The problem is that the coli half of the motor has the freewheel spline on it is the part that is not replaceable, at least not that I have seen. I know that you can buy the other half, the planetary gear side, either as an upgrade to a 500W motor or as a replacement for the 350W, for a repair. Also, I think that it is just about as expensive as replacing the whole motor with a cassette spline type 350 or 500W motor.
Both of my original fat bikes are single speeds with a 3 speed conversion. If I was buying a new bike I would definitely buy the 7 speed model with the 500W motor and the pneumatic forks upgrade. Although the price is quite a bit more than the $695 (including shipping) that I paid for my bikes. Since the city I live in is mostly flat, at least where I usually ride, a three speed fits my needs just fine.
I still haven't had time to finish my three speed conversion on both of my bikes. I have added fenders, a really nice schwinn rack on the back and an upgraded controller and LCD panel. The electronic shifter that I have designed is the last step. I have been waiting until I finished the remodel of my home shop so I have some room to work. Also I just bought a 3D printer so I could print a case for the linear stepper mechanism and arduino that shifts the derailleur. Total cost of the shifter is about $30 in parts, including the handlebar switch. As soon as I get the printer assembled and running I plan on taking some pictures and posting an instructable on how I built the shifter. The shifter might mount inside the battery box or zip tie to the wheel strut with a 7.5V lithium battery inside the battery box. I could possibly just use the 12V accessory lead to power the unit using a cheap step-down regulator for the 5V. I'll decide when I get the unit finished or at least breadboarded. If I use an outboard battery, I can still shift if I need to pedal in, low on juice
The neat thing about an electronic shifter is the ability to tweak the shift to match the cog spacing of the three speed freewheel. Once the basic shifter is finished I plan on attempting to add an automatic shift mode that will use the pedal assist sensor to automatically upshift the gears and maybe downshift to low when I stop. It's important to downshift while still moving the chainring so you are in low gear when you start up. still not sure how I might accomplish this, maybe using the break cutout switch in conjunction with the cadence sensor and a current sensor connected to the arduino. That's the cool thing about using an arduino for a controller, you can add sensors to the input pins and change the programming to match new functions.
If anyone is interested in this project, check out youtube. Search for servo controlled bike shifters. There are at least 2 videos posted, three when I get mine finished. Both of those projects use an RC model servo to do the shifting whereas I'm using a micro stepper linear servo for mine. The programming is the same, just a different power mechanism. If my method fails, I will also convert mine to a servo. The servo that they are using costs about $25-30 and the one I'm using is only $10 (including shipping). Also, since mine uses a lead screw I think that it will hold the shift position more securely than a srvo, and use less power. The stepper is really tiny and runs on 5V. I should be able to run the stepper directly off of the arduino pins without needing a stepper driver inbetween. If I do need the driver, they are really cheap, costing just a couple of bucks, each.
One last Item I would like to mention is the spare battery I installed in the battery box. I didn't have the bucks to upgrade to a larger main battery, like a lot of people do. Instead I purchased a 36V hoverboard battery off of eBay for $50. It's half the AmpHrs of my original battery but it gives me enough extra range to get home in a pinch. Kind of a spare tire. I just wired a simple switch to cut out the main battery and turn on the backup.. Since the backup is also 36V I can charge it up using my regular charger by just leaving it turned on while charging. Also the battery is small enough to easily fit into the battery box with the main battery. Definitely beats spending $300-500 on a new bigger battery, which would also cause me to want to upgrade the motor to a 500 or 750A 56V hub, and so on and you know the drill. Power makes you greedy once you step over the line. For the time being I'm happy with my bike, as is, with the planned upgrades. If I was to want more, I would probably just sell mine and get the upgraded model with the 7 speed and bigger motor.
Anyway, spring is on the way and riding time is near. Time to shake off the frost put away the cold weather gear and get ready for some fun in the sun. Yehaa!
MisterFixIt1952....Have fun Y'alll