I recommend changing themes on your website at least once per year. It keeps the creative juices flowing and will help renew your interest in a project. One problem with changing WordPress themes is that different sized thumbnails may be required, depending on the theme layout. Changing a theme will not necessarily change the size of the thumbnails, based off of a set ‘featured’ image. Thankfully, there is a plugin to help with this.
To resize your thumbnails, for a new theme, there is a plugin called Regenerate Thumbnails. This WP plugin will force a regeneration of your featured images thumbnails, to allow consistency throughout your new WordPress theme.