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Do i have to change my theme to make the plugin work?

No theme changes are needed for this plugin.

I don't want the plugin to appear in the sidebar, can i add it elsewhere ?

The current version of plugin is designed to be placed on the sidebar, in the future versions there will exist the option to be tagged directly from theme files.

How it will affect my design and blog functionality

You blog functionality and design will not be affected. When your visitors use the converter, the page will not be refreshed.