If there are any special considerations to be taken when upgrading to a specific version of the plugin they will be noted here.
Version 1.5 added a couple new features to the e-mail system. E-mails can now include the attributes of the project (similar functionality to how Workflow Notification e-mails work) and have had the templates updated. You may want to check your own System Email templates against the stock templates to be sure you are using the latest version.
Support for allowing users to reply to an e-mail to leave a comment on the project has been added. This requires some configuration to enable so be sure to check out the configuration section on that.