Contrary to what most think, I believe purpose does not have to be set in stone. It needs to evolve. The article has a nice comment on this:
Throughout the course of business, you may even find yourself needing to tweak what your “why” is. It could be that your audience’s interest in what you have to say is waning. Perhaps you were never expressing it right in the first place. The challenge is to be constantly adapting to your environment, understanding where your customer’s values lie, and then aligning your purpose or cause to those values.