You will need another instance. You can for instance, create a backup of the database, stop all services, tar the entire /opt/bitnami and copy it with the dump of the database somewhere outside the instance. Then, you can stop this instance.
Now, launch another instance with JBoss 7 but without Liferay. We do not offer the integration of Liferay with JBoss automatically, we always use tomcat for Liferay. To manage to do it yourself you can follow for instance the steps indicated in the link below:
All files you may need are in the tar. You will not extra charge by BitNami following those steps, you just need to pay the costs on launching these new image to Amazon.