Great tip! I also noted this in the megathread https://www.reddit.com/r/FortNiteMobile/comments/i4592j/v1340_megathread_return_of_the_hud_layout_tool/

If a player imported their HUD when the tool was launched for the first time, the Edit mode won't reflect what was in their old save. The easy fix is to manually re-import the legacy HUD. If they don't re-import the HUD, the edit mode will be available but will only display the Preset buttons. The "Convert Legacy HUD" button is only available if there is a free custom save slot remaining.