Post by Gnub
You could calculate a simple estimate by looking at the numbers on the Glyphs.
Glyph of Lightning Shield increases the damage by 20%
Glyph of Lightning Bolt increases the damage by 4%
If I'm not entirely wrong, you should do atleast 5 times as much damage in total with Lightning Bolt as Lightning Shield (both unglyphed), before considering changing them.
Why is that then? In order for them to reach the same potential increase in overall DPS, and the Lightning Shield Glyph is 5 times as good, Lightning Bolt should be doing 5 times as much overall damage, in order to achieve the same total percentage-wise dps-increase.
And, since your Lightning Bolt is doing less than 5 times as much as your Lightning Shield (your current Glyphing taken into consideration), it should still be worth it to keep that.
Keep in mind, this is merely weak theorycrafting, but I hope you can follow my line of thought. :)