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So each week Xur rolls a random assortment of exotics to Re-upgrade at the current 36 light for armor and 331 for weapons. For the armor the Intelligence, Discipline, and Strength all get re-rolled as well. The reason to re-roll is if the stat is higher than yours, the closer to the max it is the better the re-gen of your super/grenade/melee will be. This post on reddit has all you need to know:

Like many of you, I like to ensure maximum efficiency while playing Destiny. My Ruin Wings today have 84 STR, while the ones Xur happens to be selling today have 98 STR - a 14 point improvement. Now, Old Me would have dropped the coins immediately to get the better roll. But this morning I decided to see how much benefit upgrading would actually provide. Here's what I realized:

Several other Reddit and Bungie.net posts have provided the following information in regards to how each 3 stats affect the cooldown timers:

0 stat -
INT: 5:30
DIS: 1:00
STR: 1:09

271 stat (max) -
INT: 3:55 (assuming you aren't killing anything or stacking any buffs from your gear)
DIS: 0:27
STR: 0:31

^ If you do some simple division above, you will find how many points it takes to reduce cooldown of each ability by 1 second (I've rounded these values):

INT: 3 points per second
DIS: 8 points per second
STR: 7 points per second

Using the table immediately above, I can see that upgrading my 84 Ruin Wings to the 98 rolled version Xur is selling today works as follows:

14 point improvement -> 14 / 7 = 2 seconds

If I upgraded today then my melee cooldown is only 2 seconds faster - not that great to me when you consider what it would cost to upgrade, but YMMV. Intellect is where you get the best bang for your buck when upgrading.

Do math. Use logic. Upgrade responsibly.

TL;DR -
Every 3 points of INT reduces your super cooldown by 1 second.
Every 8 points of DIS reduces your grenade cooldown by 1 second.
Every 7 points of STR reduces your melee cooldown by 1 second.
Once you hit max stat of 271, there's really no improvement. Do the math and decide if it's worth it when choosing to upgrade your exotics from Xur.

And if you are wondering what the max values for each piece of armor are here is a nice breakdown of that as well, courtesy of reddit:
For everyone who wants to know the max rolls on exotic armor

Armor Type Split Max
Gauntlets 65 113
Helmet 73 128
Leg Armor N/A 156
Chest N/A 170
Split max are for Exotic items with more then one stat item.

This could help you to decide if the Amor items Xur is selling are THE items to buy or not.

This also applies for the items that can be upgraded. Please note that items can be re-upgraded to get the higher rolls on them.

Hope this helps

Forgive the stupid questions, but...

What's the deal with the exotic upgrades from Xur? If I upgraded something once, do I need/want to do that again?
Do the upgrades for a particular item change from time to time?
What else don't I understand?

Please enlighten me!

The upgrades Xur offers are random each week. You might not want to think of them as upgrades, but rather trade ins. Basically, you trade in the exotic you currently have, along with money and an exotic shard, and you get back the one listed. Your new exotic will not have the same stats as the one you traded; it will have the new listed stats.

You may want to trade in again if Xur is offering a better stat roll on armors.

Thanks for the replies. Very helpful... although most of the stat stuff that Up North posted from reddit went way over my head.

Considering them as "trade-ins" rather than upgrades makes a lot more sense now.