It would be next to trivial to have the representation of the label be different from the label itself, that way you could globally change not just the styling but the actual text of the label. The recommendation to use only numbers in the field and precede it by static text is ill-suited for large Word documents since search and replace on a word as common as "fig" would be a major pain since you still have to check each one for accuracy.

Note that my complaint is not uncommon in scientific writing. Different journals have different (and very rigid) guidelines for how you refer to things and you often have to change stupid things like how figures/tables/citations are represented.

In hind sight it seems like a "best" practice for using Word would be to precede the figure number by some unique made-up word like erugif (i.e., figure backwards) then right before I submit my paper to the journal I would replace all fields with static text, search on "erugif" and replace each with the appropriate FIG, Fig., Figure,... The kind of thing I would do in LaTex, but was hoping to avoid.