How long after BOGO do I have to wait to cancel an existing line

I have 4 paid and 3 free lines on Simple Choice. If I add 2 lines now to get the free line, how long do I need to wait before porting out 2 existing lines to replace these newly added lines, ultimately replacing one paid line with a free line.

Does anyone know how going below 4 paid lines would impact a grandfathered 4 for $120 simple choice plan?