Adding a custom fit tip will alter the sound. When you change the effective canal length, you change the sound signature of any universal fit IEM.
Sound signatures are created by the combination of multiple factors including the type and configuration of the balanced armature, the filters and dampeners used, and the tube length from the armature to the end of the sound bore. When you replace the silicone or foam tips with a custom fit tip, you are essentially changing the distance that the sound travels and throw off the entire tuning. Universal IEM’s serve their own purpose without needing to be converted into customs.
They are great to have on hand for back-ups or in situations when multiple guest artists will be sharing the stage. But if you are seriously considering making the transition into personal monitoring systems, invest in a set of custom in-ear monitors.