This may be a stupid question, but what happens if you improve a dropped item, say a Thunderbug Carapace Helmet to gold, would adding a glyph to it improve it in any way?
When you upgrade an item without changing the enchantment, the enchantment is also upgraded. If you put a new enchantment on it, it will not be upgraded.
As I understand it, when you improve the quality of a dropped item, the strength of the default enchantment increases along with it. So for example, if your Thunderbug helmet started out purple and you made it gold, the post-upgrade enchantment would have the strength of a gold-quality glyph. So the only reason to re-enchant an item you're upgrading is if you want to change the enchant to something besides the default.