I don’t think there’s a simple way to change them globally. When you hold down CMD (Mac) and drag up from the empty timeline area or at the end of the track and drag across only the track, you will select all of the cut points. You can apply a default audio transition.
Let’s say you set the default crossfade for 4 frames (Preferences > Timeline). Now you want to change the transition duration after you’ve already applied the 4 frame crossfades, let’s say to 48 frames. First, change the preference setting to 48. CMD+swipe across the track again in order to re-select those cuts. Now reapply the default transition, which is now 48 frames.
Yes, it’s a workaround, but it should get you what you need to do. Hopefully. I’m missing something, too.