Let people know what's changed. Often these kinds of online documents contain a lot of information and can go on for a while. When you make an update, it's likely that little has changed, so direct customers to just those areas, or highlight those changes. That way people know what to focus on and don't have to worry about the rest. If you don't direct them to the relevant changes, then they may read the whole document and get upset thinking there are more changes than you actually made.
Respond to customer concerns. Once you and your attorney have decided how you want to update your terms, the process isn't over. There's a good chance your customers will have questions about the practical applications of the new policy. Be prepared to address those issues, either in person or on a company blog, so that your customers know you care about their concerns. This also gives you an opportunity to try minimize any panic over changes to your terms of service.