Set item variations
If you want to make a set only change to a song (and keep the original song untouched), you can do this from the 'Settings' menu, selecting the SET category and clicking on 'Make a set item variation'. You will then be shown a copy of your set and asked which song should be converted to a Variation (this feature only works for songs). You can then edit this item freely without worrying about the original song changing (the app will tell you this). You will notice that the song icon in the set menu changes to a 'Variation' icon. Variations can be saved into sets without affecting the original song. Sharing the set with others allows them to also see the variation.
Variations are useful if you want to temporarily change the key of a song, or only use a small section like the chorus. If you want to repeat a song in a set, but use it in a different key, add the song twice to your set, but convert one of the entries into a variation.
Item 43 below has been converted into a set item variation: