It is fine to release apps with the SmartBeat SDK embedded while on a trial account.
If you upgrade to a paid account before the end of the trial period, you will be able to continue using the same account. For this reason, apps and crash reports from your trial period will continue to be available.
You will need to complete and send an application form to switch to a paid plan.
In the event that your account is not converted to a paid account before the trial period ends, there is no adverse affect on app performance; however, you will lose the ability to log into the web console, and will be unable to monitor crash reports.