The best time to change your business model is while you still have momentum.
A lot of small business owners are really scared to make shifts in their business. They fear they will loose what they have, they think people will see their changes as lost of focus etc.
Any good business needs to make corrections and sometimes even change the whole business model. Sometimes, your current strategy is just not attracting the right kind of customer or even revenue.
The thing is, you cannot come and make changes when things are really bad. you need to do them when your business is doing well, when the market perception is good. So you need to always keep an eye in the horizon to see what is coming and take action before the storm is too close.