Switching themes on an existing website - what are the issues?

I’ve currently got my website working fine with Stacks and a few Add-Ons, and want to switch Themes to update the look. I’m currently using Nic Cates Carbon, and want to switch to his Iris theme. What are the complications and issues with switching a Theme, assuming I want to keep all content?

Make a copy of the RW project file, rename it Beta or something else (or open your project and ‘Save As’) then work on the copy changing Themes and use Preview to see the effect and what works and what doesn’t.

What @DaveFox said. I’d also point out that the themes are very different, so expect quite a bit of manual re-adjustment. But hey, that can be a good thing - as well as switching themes you can use it as an opportunity to clean the site up, re-think any parts that may have niggled you, switch to more modern, leaner stacks and so on.