change over the subdomain to the main domain, small adjustments

I currently have two websites, my main one and then my new one which I created from scratch by creating a sub domain and building it on that. I have now finished the website and want to replace the old one with the new one whilst making sure the redirection doesn't slow the new site down. Both sites are wordpress sites. Also there is a bit of advanced coding that I wasn't sure how to do and told by the developers of the theme to get someone who is comfortable with coding etc. It is an artist website. When you click on one of the gallery categories, say drawings and then you click the link button of one of the drawings, the breadcrumbs at the top right that say "drawings" takes you to drawing posts instead of the drawing portfolio (found when clicked on the drop down "drawings" from the gallery) which looks completely different . I want the breadcrumbs to take you back to the portfolio page that you saw when you clicked on the category "drawings". subsequently I want all the breadcrumbs in the portfolio gallery pages (paintings , drawings, mixed media) to go to their category portfolio pages and not the category posts pages. Here's a link [login to view URL]