Thanks for your answers.
The solution is: you have to load the whole song "Jive at five-54swing.sty demo", then you can edit the style (then rename and save the song)and the 5/4 count-in is preserved. If you load only the style 54_swing.sty, the 5/4 count-in is lost, when you edit it.
I don't know why that is so, but for me the problem is solved.