Liquid Rhythm (up to 1.4.5), Liquid Music (1.5.0-1.7.0), and Liquid Music 1.8.0 have three independent licenses. As a customer in any category, you may install a trial of another version and revert back to your original license type before purchasing an upgraded license. However, you cannot have multiple versions installed concurrently. A complete uninstall is strongly recommended when changing between versions; instructions here: macOS. Windows. Be sure to not remove your license when completing the optional step listed in the uninstall article.
You can download Liquid Music 1.8.0 and register a free trial, it will replace Liquid Rhythm but your original Liquid Rhythm license will remain intact (as long as you did not remove it when performing the manual uninstall process above). At any time you can uninstall Liquid Music 1.8.0 and re-install Liquid Rhythm 1.4.x (or earlier) to revert back to your old license. You do not need to re-register Liquid Rhythm after doing so.