Can RBR 750/860/960 restore from RBR50's backup file ?

FURRYe38 is correct. Customers have asked for a method to migrate a configuration to a newer system for some time. There are significant differences from one Orbi product line to another. (Heck: the configuration file for the RBR50 has changed over the years as features are added or deleted.) There are a couple of steps I would recommend:

1. Screen shot the entire configuration (every page), and save it to a Word or pdf file which can be printed out for reference.  It may take more than one screen shot for some of the pages, which can be too long to fit on one screen.
2. If you use Windows, download a copy of this utility:
   https://github.com/Fysac/orbicfg 
   I have used it "decode" and display the contents of the configuration file into plain text.  My practice is to sort the test file so that similar entries appear together.
3. When the new system arrives set up the router before it gets connected to the internet. (Do  not bother with any satellites.) The default WiFi will no doubt be different than the WiFi on the existing system, so they both can be active at the same time.  None of your devices will attempt to connect to the new system.  Connect a computer (PC, Mac, laptop) to the new system and go through the configuration process, ignoring any suggestions to change the WiFi name (SSID).  There is no need for haste because then entire WiFi network is still functioning.
4. After the router has been configured, register the new system with Netgear on https://my.netgear.com   This will start your 90 days of 'complimentary support'.
5. Power off the old router, move the Internet Service Provider (ISP) connection to the new router, and power it up.
6. Go into the new router web interface and change the WiFi SSID/password to match (exactly) what was on the existing system.  When the change takes effect, every device in the house will connect the same way they did to the previous system.  If there is supposed to be a Guest network, that can be set as well.  Go through the "Add Orbi Satellite" process to connect new satellites and relocate them to wherever they are supposed to go.
7. I have no clue how a customer would move an Armor or Parental Controls subscription from the existing Orbi to a new system.  Have never activated either of those products.
8. Do not panic.  If disaster strikes and it all goes to s**t, the old system can be powered back up and everything goes back to normal while you call Netgear support.