I recently was asked this question: “I have the TwentyTen theme that is not responsive. Is it difficult to convert to a responsive theme or do I need to start from scratch?”
Here’s my answer:
It depends how much you have customized the theme. If you are using a default WordPress theme pretty much “out of the box”, it should be simple to switch to a new theme. The new Customizer in the backend will let you switch themes and see how it is going to look before you commit to a new theme and click that save button. Just keep in mind that you will probably have to make sure that your custom menu is set up where it should be and that any widgets that you are using are put back in the place you want them. If you are only using the Primary Sidebar widget area, this shouldn’t be a problem, but if you are using custom widget areas, you many find your widgets down below in your Inactive Widgets area and have to drag and drop them to your new widget area. Otherwise, most of your settings should stay the same. All your pages, posts, and media library will be there as before. And any functionality you had through plugins should remain, as long as the plugin is compatible with your new theme. If you are using another theme that is using Custom Post Types enabled through the theme, those special pages and items will be lost. (Of course, you can always get them back by re-enabling the old theme.)
Please post any further questions you have below. Good luck!