Is there any way for content transformation in alfresco without using any rule, so the existing contents will be transformed without manually uploading them again
If you already have a rule on the folder you can tell the rule to fire without re-uploading the documents. Just go into "Manage Rules", click on the rule, then click "Run Rules…".
Alternatively, in the old Explorer user interface there is a link called "Run Action". You can click that, select the transform action, set your parameters, and then run the action. Unfortunately this does not exist in Alfresco Share out-of-the-box.
Other than those two options there is no easy way for an end-user to do it. However, if you have something like Florian Maul's JavaScript Console installed, you can go to the document or folder in Alfresco Share and click "Use in JavaScript Console". From there you can use JavaScript code to instantiate one or more actions and execute them against the folder or documents.
