You give Embers to blacksmiths so that they can then reinforce weapons further, they aren't consumed. You use Titanite Shards, Large and Small, Chunks and finally a Slab to reinforce weapons.
Reverting takes the weapon back one modification, so from +6-10 back to +5. You lose all of the materials that you used to upgrade, but you can reset and start over to down a better upgrade path.
Since you have an Iaito, modify it to +6 with the blacksmith in Undead Parish. Farm some Large titanite shards, or grind souls and buy them from the blacksmith that made your Uchi Lightning. Try to get it to +10, then compare it to your Lightning Uchi for damage output. Stop using Titanite Chunks on your Lightning Uchi, save them to eventually use on the Iaito, when you find the Very large ember, which can take the Iaito to +11, and then on its way to +15.
If you are going to level Dexterity and/or Strength you need to work toward +15 to benefit from scaling.
edit: lol, you are set then, use the Iaito. It is way better than the Uchi if you have around 30 or more Dex. Get your dex to 40 to get the best scaling.