Sorry – I should have been a little more clear on this. When I talked about related portfolio entries, I meant something like this –
https://docs.google.com/file/d/0B9j6FQo4cSQFMV9Dc01QZ1gxMkE/edit?usp=drivesdk
and you can use the post_snippets shortcode inside the portfolio entry content to generate these related entries by specifying the filter for current portfolio category (terms) in the shortcode as linked here –
https://www.livemeshthemes.com/support/forums/topic/getting-portfolio-with-slugs/#post-1772
Alternatively, you can have a button replace the next and previous links. This button would navigate back to the page which lists the portfolio items for that category. Just like the Launch project or View Website button, you would have an additional button which helps the user navigate back to the page from which the portfolio entry was opened.
The next and previous links cannot be changed to navigate among the related entries only. The code posted above just hides these next and previous links. Hope this helps.