You simply move up the half-merge of each lane to suit. It's rather clearly explained in the diagram, where you can see it does end in one and only one lane act

You simply move up the half-merge of each lane to suit. It's rather clearly explained in the diagram, where you can see it does end in one and only one lane actively in use. You'd do the "merge half of each lane" part while the road still supports two lanes, just as you already have to account for with current temporary traffic control devices.