You can change the tempo once you have Ableton Link enabled ... which version of rekordbox are you using?
Slave mode on Ableton Link
I would like to request the implementation of slave mode in ableton link's sync.
Its functioning only as a master greatly limits the DJ's actions. In Traktor you can change the speed of tracks using the BPM fader.
I believe that with this change you will embrace a large community that likes to do live performance because in this way its use is very limited and few use it.
the following message appears:
Invalid operation. TEMPO slider cannot be operated with the hardware on decks whit ableton link enabled.