When the euphonium was first invented about 200 years ago they did not understand anything about the ergonomics back then.
Today we do understand the ergonomic challenges involved in this. Furthermore, modern horns have become a lot heavier; therefore, that old irrational tradition of holding your instrument with the everlasting static tension especially when played standing up should be questioned.
You need some time to readjust your perception about euphonium playing. You need a moment to get used to the different feeling of the nearly weightless instrument. You also need to practise how to handle the horn, empty the water etc. Everything cannot obviously be done exactly the same way as before, but the new way is not any worse, just slightly different. After the learning period, you will enjoy playing with less tension.