You can pay off or upgrade the "buy one" device as soon as the bill credits start on the second device (the “get one” device) and you will still keep your BOGO credits.
However, if you pay off or do an early upgrade for the second (the “get one” device), you lose any remaining BOGO credits.
To get all 24 BOGO bill credits, the "buy one" 1 device’s line must be active for at least 6 months and the "get one" device's line must stay active for 24 months.