Unlike a lot of things like debugging and region changing, accessing the test mode actually
doesn't involve taking your Tamagotchi apart at all. It's designed to be a test that they can use in the factory and so by its very nature it's not intended to be something that harms your device.
However, I think a disclaimer should probably always follow any talk about ROM testing because it is still a developer feature that is
not intended for the general public to use, but it doesn't involve any messing around with the hardware of the device, and I have seen literally hundreds of tests without any reports of damage to the device.
I think there's a few important things to consider when accessing the test mode:
- Is the device debugged? In case you're uninitiated, debugging involves actually taking the device apart and messing with the circuit board. You can tell if a device is debugged as it'll have a "hyperspeed" mode activated by pressing the C button, which causes time on the device to travel a lot faster than normal. There were reports from years ago that ROM testing a debugged device could break it, but these rumours seem to be entirely unsubstantiated and recent attempts to ROM test a debugged device has shown that in reality it is actually impossible to access the test mode on such a device.
- Are you okay with losing your save data? Accessing the test mode by itself has no impact on your data and after accessing it you can usually pick up from where you left off, but the exception is that if you reach the "ROM TEST" part of the test mode your save data will be wiped and you'll have to start from the beginning.
When entering the test mode the first few screens are intended to test the pixels and icons on the device and identify those that don't work. This is then followed by a screen showing the firmware version of the device, and after pressing a button on this screen it enters the ROM test part of the test mode. If you don't wish to lose any data then you can perform the test up until the version number screen, but I wouldn't risk it in case you accidentally press a button on this screen.
If you do lose your data, no data from the software of the device is lost - it just simply acts as though you've got a brand new device, and you'll need to start from the first generation. It's basically like choosing the "RESET" option on the DOWNLOAD / RESET screen.
So basically, if it's something you're considering, take the above into account, make sure it's definitely something you want to try, and maybe try it on a cheap device that you don't mind resetting first so you understand how it works.
Since you mentioned the "numbers" involved I'll explain a bit - the "version number screen" I mentioned is a screen showing the version of the Tamagotchi firmware that your device is using. Two devices might both be the "V2", but they could have different firmware versions, indicating that they'll be subtly different in a few different ways. Here are the known version numbers corresponding to the V1 and the V2:
V1
0.0 - Tamagotchi Plus (Japan)
0.1 - Tamagotchi Plus GLAY Expo Edition (Japan)
2.0 - Tamagotchi Connexion, first wave (Europe, Australia)
2.1 - Tamagotchi Connexion, later waves (Europe, Australia)
2.3 - Tamagotchi Connexion, unknown waves (Europe, Australia)
4.0 - Tamagotchi Connection, first wave (USA)
4.1 - Tamagotchi Connection, later first wave devices (USA)
4.2 - Tamagotchi Connection, later waves (USA)
V2
A.2 - Tamagotchi Connection V2, first wave (USA devices that were shipped to Europe and Canada before the eventual release of the V2 in the USA)
A.3 - Tamagotchi Connexion V2, first wave (Europe)
A.4 - Tamagotchi Connection V2, later waves (USA)
A.5 - Tamagotchi Connexion V2, later waves (Europe, Australia)
A.8 - Tamagotchi Connection V2, Spanish version (South America)
A2 H - Tamagotchi Connection V2 PUMA Edition (Europe)
There are likely more versions than those listed above that have not yet been discovered.
There's a video of the test mode in action
here showing a V4.5 if you want to see what it looks like. The thread TamaMum linked above should also contain more details if you want more information.
Hope this helps!