If an item has a UV (like low fiend bonus) and you upgrade it you can keep that UV bonus.
However, when does this options occur? More importantly, can i upgrade it, see if it gets a better UV and then choose which 1 i keep, or do i have to forgo the opportunity to get a better UV in order to keep the old one?
You revoke the right to a new UV when you carry over the old UV. This option is available in the crafting UI, when selecting the item you are going to craft, under the materials list there is a slot for the item you intend to upgrade, and under that slot, a checkbox to carry over UV(if the item inserted into the slot has a UV).