Just had a look at the instructions for my BL3500.
It doesn't mention any procedure for swapping between MCL & Beolink.
My guess is to power down your BS3000 & BL3500, connect the masterlink cable to both products and switch the power back on. Switch the BS3000 on first and then the BL3500. I think the 3500 will recognise that it is now on Masterlink automatically.
From memory the clock can take a little time before it displays.
Hope this helps
Peter