This CAN be changed on the back end (outside of a game/scenario) by modding, or if you know 'where to look' r.e. a unit you designed in a previous game, but not on the front end by players.
Reason: When the game checks for upgradeable equipment, it uses one variable. For almost all units, this is Defense_Pierce. This makes sense in most cases, but with the Soldier's Gloves and Soldier's Boots, these provide a different bonus (accuracy/initiative). As such, they provide Defense_Pierce of 0, so the game looks to upgrade this to a higher Defense_Pierce value.
You can manually edit the unit design file to have the game look for Accuracy, Moves, Initiative, etc., but not during unit design. If you are happy with your unit design, and want it to stick around for a while/multiple games, you can go into the mydocuments/mygames/LegendaryHeroes/Units folder and change a specific 'custom' unit design, if you are feeling brave...
Open up your Unit file in notepad (or some .xml editor) and look for these entries:
to either: UnitStat_Moves
BTW, My using of colors here is for emphasis only!!! you don't need colored text, and probably should use the same color of text (black) when you make the change. Some .xml editors assign their own colors to various classes of entries of course. Also, it might not hurt to copy the file first and save it in a safe place NOT in this folder, in case you inadvertently mess up the file (delete a > or something)
Remember to save your file as 'unitname'.xml, with the .xml extension. Saving it as .txt will create a new text file, and the game looks for .xml files.
Said changes will NOT take effect until you start a new game, so this won't help at all with a current savegame.
If for some reason someone were introduce a new set of boots to Unit Design that gives, say, +2 to Combat Speed, with the Comparison attribute set to Combat Speed, said upgrade path would upgrade the boots to the 'faster' boots. Food for thought...
Again, this is not an ideal solution, but if you have a favorite unit design, which you are willing to change these upgrade paths for future games, this is how you do it. Of course, if you edit the unit ingame using the Design function, said variables may 'default' back to Defense_Pierce, so keep this in mind...
Another thing you can try is to NOT set Forearms or Boots to automatically upgrade while you are designing the unit, but looking at my unit files, it appears that said values were included in the UpgradeDefs anyways... However, if this works for you, it'll save the need to edit on the back end later! I'll test this the next time I design a unit and if it works, I'll post up here again. If anyone else has had success with this (blocking the upgrade path in Unit Design), kindly share.