You can absolutely change your site’s Terms and Conditions — and you probably should, periodically. Any time you change something on your site that might affect your users, you should update the Terms and Conditions to reflect those changes.
You might need to update your Terms and Conditions for the following reasons:
Implementing new software
Starting to sell goods and services
Complying with the legal requirements of new laws
Creating an easier-to-read or more descriptive set of conditions
Adding new website functionalities
Note that any time you make changes to your Terms and Conditions, you should notify your users about the changes before they occur.
Failing to update your Terms and Conditions when necessary can result in negative consequences for your organisation. If your changes impact a user’s rights, you might be opening up your organisation to otherwise avoidable lawsuits. If a user isn’t aware of these changes, they can argue that they never agreed to them.