If your existing theme is from the Shopify theme store, then there are some benefits included in that one-time fee you paid. Shopify themes come with unlimited free updates to the latest version of the theme. This is useful because most of the theme developers are adding new functionality to their themes, and overtime; this can be useful. However, updating the theme to the latest version can be a time-consuming task. When you receive the latest theme version, you're receiving a brand new file with the theme, not set up the way your existing copy of the theme file is setup.
Out of the Sandbox and Pixel Union Themes
One of the benefits of selecting an Out of the Sandbox theme or Pixel Union theme is these two companies have an app that transfers all the settings from your existing theme to the latest version of the theme automatically. If you have custom code added to the theme, the app will recognize that and transfer that over as well. There is a free and paid version of the app. The paid version is for store owners that have custom code added into the theme, and they want that auto moved to the new version. Otherwise, the app is free to use and update your theme anytime.
Updating all other Shopify themes
If your updating other Shopify themes both from the Shopify theme store and different themes on the internet through companies like Theme Forest, then it is a manual process to update. You will need to set up the theme 100% from the ground up. This means all the graphics, category banners on the home page, sections, settings, layouts, content, fonts, colors, and more. You're essentially starting over, except since you already have all the settings in place, it is a bit faster to copy-paste and match the settings if you have the old and new side by side on your screen. That is how we do it if you're looking to update, and you want Pelican Commerce to do it for you.
If you need help updating your theme then reach out to us anytime.