Before when I added a +Stat Enchant it would add that chant in addition to what was built into an item.
I remeber having some Boots from ZF with say +14 or so Agi and having an enchanter add +5 Agi to that. It would then show in green.
Last night I bought some new boots for my mage that had +3 Stamina built in. I went to enchant them to add +3 Stamina and it asked me if I would like to replace the +3 Stamina. I gave it a try for expirementation's sake.
It just replaced the +3 white stat rather than adding to it.
This may be a well known thing from PTR but it was new to me.