Changing a Permalink Structure

I wonder if you can help with an important issue.

Our site is using a permalink setup as follows:

http://www.domain.co.uk/%post_title%

We would like to change it to:

http://www.domain.co.uk/%category%/%sub_category%/%post_title%

Maybe something like:

http://www.domain.co.uk/news/category/post-within-sub-category

How can I achieve this permalink setup successfully without losing link juice from old permalinks?

Thank you 🙂