Yes, you can always upgrade your plan at any time from your Bluehost dashboard or you can ask customer support. However, it would be better if you pick the right plan from day one as when you upgrade the Bluehost plan, you will not get the discounted pricing. You will be charged regular pricing which adds up a lot of costs. If you are not sure which Bluehost plan to buy, then simply pick the Plus plan which is idle for hosting unlimited websites.