This is important. Once you have completed the transfer do all the things you used to do in Lightroom. You will find the odd thing still needs to be done. And best you find all these things out while you are sorting out all this stuff.
I had to add the new catalogue as the chosen catalogue to sync with Lightroom Mobile.
Never though of that one! I thought I had lost the facility to sync Lightroom Mobile as there was no indication of how to do this, but I found a dialogue box which quickly sorted this.
So all you do is choose the new catalogue to sync, and then you have to choose the collections you want to sync. And then let it do its thing, which it just has.
I am going to look into Collections in Adobe Lightroom CC and will write about this in the future.
This and Smart Previews are transforming my workflow for the better. Much better!