Since open source you can flash whatever you like and then flash the original FW back whenever you like.
No one can proof that you used different FW in the meantime.
If you don’t have the original FW you can ask for it and there is a duty for Enertion to send it to you or offer a download. They need to do that for another 3 years after last distribution of software or hardware with that FW on.
You should always use the latest VESC-Tool and FW anyway. There are some safety improvements in VESC-Tool that do not come with the older stuff. Benjamin highly recommends not to use the old BLDC-Tool any longer. It doesn’t get any critical safety updates and is outdated.
One feature that is very important to have is Acceleration temperature decrease. Without that feature your brakes can drop out at when your motors reach critical temperatures. The ESC will cut back the power when the motors overheat. It will also cut back the power of the brakes! In consequence you will not have any brakes when needed. Acceleration temperature decrease will give the brakes headroom to work at any time. Especially with hub motors this feature can be your life saver! Hubs tend to overheat faster and the risk of ending up without brakes is very high.
Since sources for the Enertion distribution of BLDC-Tool are still non existent, the community has no chance to work such a feature into that software or find existing issues. Open Source should work in a way that many eyes look at the code and weed out the weaknesses, so that it gets better and better, safer and safer.