When the update launches, the embers, motes, and figments in your wallet will not change unless you have more than the cap (10,000). If you have more than the cap, you will lose any excess, so you should spend them now.
If you can, you should have 10,000 embers in your wallet at launch. You won't want to spend them until you reach 130 (the new cap), but you probably won't be able to earn any until then either. You might as well start with some if you can.
Level 120 gear (in other words, all current gear) that disenchants into embers now will be changed to disenchant into motes. It looks like coffer ember barter pieces that currently produce 400 embers will produce 320 motes. The 120 quests that reward embers (weekly exchanges for Longbeard Marks or Gulmarks, wrappers for the Gladdenmere dailies) will instead reward motes (and not as many).