I'd change the Reduction Gearbox / Differential fluid early, then never again. This is to get out break-in metal as the gear surfaces polish down a bit when new. About 1,000 miles or so, whenever you can. .... You don't really have to touch it, only if you plan on keeping an i3 past 100,000 miles or so, to cut down the wear rate by getting out tiny steel particles floating around in the oil the magnet didn't catch. Sometimes iron particles can be washed back off the magnetic drain plug from fluid mass flow. Also, those magnets always allow some iron to circulate, seen from oil analysis done on many gearboxes, trannys, and diffs over the years.
Unlike all other EV gear reduction & diff boxes out there now, BMW specs some thicker stuff. The part number the OP linked to is 75w-85, although it is GL-4 which does at least make it somewhat similar to the lighter visc ATF fluid all other EV makers use. Bolt, Leaf, Tesla, Ford EVs, Kia, Hyundai, all use thinner standard ATF red fluids which look about like a 0w-16 motor oil in viscosity anyway (additive package is diff of course). Quite thin, for better energy efficiency & less heat generated.
I'd stick with the BMW-recommended fluid for sure, a GL-4 75w-85 here.
I've changed this on a couple of EV's recently, other brands, and from the OP's link to the part (see that original post), it does say it has a magnetic drain plug. (Nissan Leafs have a huge magnetic drain plug, and the same for the Fill Plug too, both of which catch a lot of iron particles from normal break-in & running wear, even at 1,500 miles on a new-ish Leaf!)
One would need to simply identify where the Fill Plug is, and where the Drain Plug is. Usually an aluminum or copper crush washer of the right size is needed. Actually the Service Manual should be consulted to find out if there is anything Unusual or Special to do, as BMW is known for over-complicating the simple stuff.
If you do change it yourself, which should be easy (probably!), then don't tell BMW you did, since they do not require you to change it during the long warranty period anyway.
If you do NOT change this fluid, do look for any leaks once every couple of years, or even check the level at the Fill Plug. At least look for leaks. .......These things are almost always designed to get the level right by opening up the fill plug & adding some until it starts running out the Fill Plug, although some are picky about what temperature you do it at, so reading a Service Manual would reveal how easy or hard this really is !!