Additionally, you don't necessarily know what the customer is thinking. For example, some of the lifetime purchases I've made are because I see a deal as well as a desire to support the person building the product.
To be fair I do usually see the prices increase over time, so I understand your concern. Even so, I don't think you should worry about this or take any pro-active action. It's your right to adjust pricing as you see fit. If it bothers you, have a plan in reserve to provide a partial refund if any customer asks what is happening. The reality is very few people will probably even notice.