Thanks for the reply. Technically, my previous theme is calling the content from single.php file. While the new one is not.
I know that there are some coding works to achieve this in WP, however I am not a capable of doing so. This is the reason why I am seeking help for those who can understand how WP templating works. I have tried reading and applying some techniques from WP codex, but I cannot get this thing to work.
I am using this simply to apply different themes/layout for specific post. Afterall I am not using TAG function of WP. Category is already set for each articles so I may not be able to use the 'Category Template' function. Besides I have posts in the same Category that needs to be shown in different layout.
My question is basically on editing or using the right code for the get_template_part function to call different file as per Tag of the post.