To change COM port assignments for a Digi Neo, go to Windows Device Manager, expand the MultiPortSerial Adapters category, double-click the Neo in question, then click the Advanced tab.
COM port numbers for individual ports may be changed by selecting a single port at the top-left then inputting the new COM number at the top-right, and then clicking the "Set COM Number" button. Click OK.
To change all of the NEO COM port numbers sequentially, select the first port at the top-left then input the new COM number of the first port in the COM field in the middle-right, and then click the "Enumerate All Ports" button. Click OK.