Well.. it is hard to decide since I am stuck on the same shoes. But to answer your question, sub templates from V2.6.8 will work in version 3 since v3 has legacy layer for backward comptability. However, there has been a few changes to the template side of things in v3 and if you want to keep it clean and do things the V3 way, then you would eventually need to update your template for V3. So in other words, old templates should work but it is better to update them for V3 eventually.
I have the same issue as you. Since V3 seems to be going to take "forever", I now have to spend all my time developing my K2 template for V2.6.8 first (since I am creating from sratch) and then when V3 is released, I have to do all that once again. Not very happy.. wished v3 development wasnt crawling like a snail with a broken foot.