Upgrade question

I have a bow with the following mods :

+17% damage (vs hex foes)
Enchantments lasts 17% longer.

I want to put a +5 armor upgrade in to replace the +17% mod. However , I am afraid it may replace the enchantment mod instead. Does anyone know which mod it will replace , and how would I be able to tell which mod upgrades will replace in the future?

http://www.gwonline.net/page.php?p=125

Read this. The Prefix modifiers should replace the prefixes and suffix the suffixes.

Link was very helpful , many thanks.