From what I have been told, there's much more to it than just increasing a define to get maximum upgrades working.
For piercing it might be true, that you just have to change one define, for normal upgrade limit it's definitely not the case.
As for the safe upgrade, this is set in the WndField. CWndSmeltSafety::OnChildNotify
The define SMELT_MAX is used to limiting safe upgrade. I can't tell you if other things bug out if you just change it, but I think they might.
I believe in order to change the max general upgrade, you would have to change the ItemUpgrade.lua reading, the upgrade windows, some defines and exptable.inc for the attropt set effects.
Hope that info helped.