So, I got one of these cheap 4072C to play and then decided to see if it was possible to hack it from its original 70MHz to the 200MHz maximum bandwidth. I successfully managed to re-config it and found that it is rather simple, it is very similar to the other Hantek hacks with just a small different detail. Here is how I did it, if anyone is interested:
1 - You need to open the case to get access to the serial interface on the PCB, it is located beside the usb connector and the pinout is the same as the others, from the usb connector the pins are: unused, RX, TX , GND and 3v3 (not needed).
2 - Connect the cables to an USB-TTL converter and on your pc open the terminal emulator of your choice (I use Poderosa 4). Open a serial connection with the configuration 115200bps and 8-n-1.
3 - Turn on the oscilloscope and wait for the boot. Before change anything I did a full backup using the tool nandbackup_V2 provided by @fremen67
here (just extract the files to an usb flash drive, plug it to the scope and execute the script, i did ./nandbackup_1.00.2_OOB and ./nandbackup_1.00.2 just in case, it will take about 15min each).
4 - Now, to change the bandwidth you need edit just one line in the file i2c.log located in / (I would recommend you to make a copy of it to your flash drive as well). Open the file with the vi editor and change the last line: where you see [bw] 70 to [bw] 200 to get the 200MHz or [bw] 100 to 100MHz. Save and close.
5 - Now reboot the scope typing the command reboot [enter] (if you just turn off and on the scope, it won't work), and that's it
Opening the menu Utility -> System Info you should see the model changed to DSO4202C.
To be honest, I am not sure if there is really any difference among the models. I don't have a function generator or other equipment capable of test the full bandwidth, my hacked HDG2002B is useful up to just about 50/60 MHz so I couldn't see any practical change after the upgrade, if anyone could test it and confirm, I would be grateful.
Some pictures