So I have been searching for what is probably a simple answer to a simple question, but haven't found anything conclusive. More complex answers to complex questions.. And yes, I am pretty noob-ish, so forgive me for that.
I have a wiikey 1 with firmware 1.9s. current wii firmware is 3.1E. Not into homebrew, wads, twilight, or anything like that. Never really had the need.
Is the upgrade to 4.0E safe? or will there be disastrous consequences - acting like a non-modded wii, or bricking it in some way? Or will it suddenly burst into flames? Morph into a blood-thirsty Transformer robot?
The upgrade is perfectly fine. Theres some new games you wont be able to play correctly id you dont upgrade such as Punch Out and Night at the Museum. They both require the 4.0 update, or the work around with homebrew which you said you wont want. Heres the catch if you go ahead and update you may run into issues later on if you decide to install homebrew. Theres a couple of How-To's out there telling you what you need in place before the upgrade.
If you want to have homebrew then simply install ios60 and cios v7. Then upgrade the wii through the regular internet channel. This is known to be safe because the updates themselves replace everything. I would extremly avoid using a custom made upgrader for many reasons.