I have a website that will need to be fully redesigned. Only meta/title will stay. The rest of the website will be completely changed(description,images,etc). It will have more content(3-4x) and have a much better on-page SEO optimization. With changes so drastic will it be dropping in SEO rankings for a period of time? Certainly rankings for a few images the site has will be completely lost,but I am trying to preserve a few pages of the old website and transfer them to the new one with different design.
Try to understand this. Google and other search engines will start seeing the website differently. The new design might push the content downwards on the page. It may push the links etc down or up etc. So this "may" have some differences in search rankings for a "short" while, but as you mentioned that you are going to have more and better content so your site might get seen as an authority site as well which will help improve rankings. By saying down and up I don't mean visually down, but down or up for search engines. I think there are tools you can use to see how search engines see your website. Use one of those to see how much difference is coming up. Best wishes
If you are just changing the design but using the same urls and inbound links, then it wont affect your ranking.
Your changes will help with SEO, you'll probably boost your keyword rankings. More content, properly labelling images, and a better design will in fact keep users on your pages longer (also helping with SEO). Goodluck!
There is one peculiarity: when you change the design of your website, the number of clicks on some links can decrease, so the weight that is given to the keyword through anchor links will also decrease. Advice: if SEO rankings will dropp in, then check the number of clicks on the appropriate Links in google analitycs.
For my experience, I think changing of design doesn't drop my rankings at all when my blog was still alive. The rankings doesn't matter on the website design, but on the way you optimize your content, and how quality, relevant and original is your content for visitors to read.
It is usual to take effect on rankings when changes are implemented within the website. 301 redirect is the best way to shift your pages and pass it link juices.
About 1-2 years ago I completely change design of a website, including urls, image location, etc etc.. It only need about 2 weeks to recover the rank after downfall to nowhere in google.. So, if you have a good rank and good back links before, your site is will back on google position..
For some regular users, it might be disappointing to see a different design that their minds aren't in agreement with but if your content is great in quality then that's something user would get used to. Take the example of facebook, no matter how much people dislike the new changes, they're gonna keep coming.
The new design will not change or harm your ranking. The URL structure can do it, but for example if you change the background from red to blue, nothing to worry about (I know you are talking about a big change, but I just try to describe you!)