If you mean changing the tempo of the playback, AFAIK the Tempo setting on most keyboards affects whatever you're playing-- the auto accompaniments, the arpeggios, the songs, etc.-- so that should work. Note that each auto accompaniment, arpeggio, song, etc. usually has a default tempo that will be used whenever you select that particular auto accompaniment, arpeggio, song, etc., but that shouldn't prevent you from changing the tempo of the playback if you want.
But if you mean changing the tempo and saving it such that it becomes the new default tempo for that auto accompaniment, arpeggio, song, etc., then that may or may not be possible, depending on the capabilities of the specific model as well as what it is you're trying to change. For instance, some models let you change the parameters of preset voices, effects, arpeggios, and auto accompaniments, then save the modifications as a user voice, user effect, user arpeggio, or user auto accompaniment. But I think most keyboards usually have copy protection for the internal songs that prevent you from changing them and saving the changes. On the other gand, if the song is a user song-- one that you recorded yourself-- then I think you should be able to change the tempo of the song and save it.
Do you already have a WK-7600, or are you asking because you're interested in getting one and want to be sure whether it can do the things you want it to?