As far as I know, you’d have to use a combination of built-in functions to achieve this. If there is a plugin, you’d have to research for “Bulk parent” or “Bulk re-attachment”.
Unless you have a logic connecting those posts, I think you’d have to do it post by post. Be it with PHPMyAdmin (changing the post_parent column values for all post_type with attachment value). Be it with an already existent or custom built plugin.
One solution is creating a Meta Box, where you’d enter the target post ID and all attachments from the current post would be moved to the new post.
It can be built researching the following queries:
There is a very nice and small technique from WPEngineer to add a Re-attach column in the Media Library (wp-admin/upload.php), that may be of interest:
Small Extension for the Media-Library
[…] images can be easily reused. A link will be created so you can access the image directly and there is no need to upload the image again to your new post. Further information can be found in this post.