I'm almost ready to go live with an SaaS company that I have been developing. It's a php/mysql driven web app that will cater to mid-enterprise level businesses. The way I've gotten my application setup is each account gets there own database, but everyone uses the same php script.

I'm very comfortable with the stability of the code now, but there are features that I would like to add in the future if customers are interested. Where i'm worried is when I have a new feature to implement (which would require new code and mysql table changes). I'm not sure how to smoothly and properly implement changes to an SaaS app without taking customers off-line for a period of time.

Any insight?