I have a router with one satellite (RBS50V2) and I wish to extend with additional satellites.

Yes, they are interchangeable.

Thanks for the reminder that you have two "sets" (router and satellites).  That makes it much easier to determine the firmware release.

Since an Orbi system will function without an internet connection, you can do this:

• Plug in the new purchase.
• Connect to it with either an Ethernet connection or using the WiFi credentials printed on the product label (bottom of the case).
• Open a web browser to http://orbilogin.net or to http://192.168.1.1 
• Pay no attention to any warnings about the connection being "insecure" and select an option on the web browser to "proceed to this address anyway."
• The firmware release appears in the upper right portion of the screen.
• Power up the two newly purchased satellites and see if they connect automatically to the new router.
• If they do not appear in the Attached Devices screen in 5 minutes, select the Home Page menu item, "Add Orbi Satellite" and go through the process to add each satellite.
• In a few minutes, the Attached Devices screen will show the two new satellites with their IP address.
• Connect to each satellite with a web browser and record the firmware release on each.

My guess is that the existing system shows firmware v2.7.4.24 on the router and satellites and the newly purchased system shows a different version.