I believe you'll be able to use all of your old weapons and gear for all content not included in TTK. So, you can still use the same weapons and armor you've been perfecting up until now in VoG, PoE, CE, etc... But they won't be as effective in TTK content or things like Iron Banner or Trials. Basically, I don't think all your work is invalidated and your time wasted, because you can probably use all of your old stuff for the old content.