I have the Arturia Keylab MkII 61 which has pretty much the same controls as the 88. The center preset controls are not programable. They do send out some MIDI signals in DAW mode & Analog Lab mode. But it would be confusing to use these for what you want to do.
I simply programmed 4 buttons in 1 row.
Button 9 = Previous Rackspace
10= Next Rackspace
11= Previous Variation
12= Next Variation
I did have to change the options for the buttons at the bottom of the Arturia software page (see photo).
Assign these in your Global MIDI Options and you’re good to go.