Let us know how you like it once you need to do it....especially on a rocking boat between dives!
Ignoring the O2 setting and using air settings for all dives is certainly acceptable -- for recreational diving where you don't really need to track O2 exposure -- but does not allow taking advantage of the occasional desire for a longer-than-air-NDL dive or a shorter SI. I know YOU know that, but new divers might not, and this is in the New Diver forum. The point is, Nitrox has advantages other than less N2 uptake, which some training agencies argue is not "safer" because how can you be safer than safe?
We are going severely off topic if we start talking about Nitrox advantages for rec diving (very few, for me) and drawbacks (several and severe, as it limits significantly MOD and requires additional skills, procedures for being sure of the O2 percentage, etc.) - for what in change? A few more minutes of bottom time, after which, when you emerge, your slower tissues are more loaded by nitrogen than emerging after hitting the NDL with air?
As said, I had never the need to change the O2 percentage on my Leonardo, yet.
But according to the manual, it appears barely simple (everything is very simple with this device, as it can do just a small fraction of things you can do with other computers).
Judge yourself. The manual is here:
https://www.cressi.com/easyUp/file/instructions/IB_LEONARDO_EN.pdf
The instruction for setting the Nitrox O2 percentage is on page 20:
Just tried it, less than 30 seconds.
Is this really more complex than with other computers? I do not know, the only other computer I used in the past was a very old Aladin, and at that time there was no Nitrox...