As an experienced web developer, especially in my exposure to E-commerce and CMS-based websites, I have acquired thorough knowledge and hands-on expertise in Umbraco versions 8 to 10. Over the past four years, my team and I have successfully carried out numerous upgrades that required seamless transitioning between Umbraco versions, just like what your project entails.
Beyond mere familiarity with Umbraco, I have a solid grasp of its extensive backend functionalities and potential customizations — this includes those that may already be implemented in your existing Umbraco version 8. This will ensure that your upgrade process is not only smooth but also leaves no room for any loss in data or relevant business information.
Not to worry about compatibility challenges or necessary updates for your customizations - I've got you covered. Alongside my strong capabilities in JavaScript, which would be vital for this upgrading process, I'm committed to ensuring all current customizations are properly validated and made compatible well within the new Umbraco version 10, adhering strictly to your expected time frame and budget delineations. With me on your side, you can rest assured that your Umbraco upgrade will be handled by a professional who would not compromise quality while working within an efficient cost approach!