When I decide the poly crosses need to be restrung, I mount the racquet in my 6 point machine as near as possible with the mounting clamp settings like they were when I finished the last restring.
I then carefully cut the crosses out, being sure not to nick the gut mains. At this point, I have a main only string bed just like it was when I finished putting in the mains for the first time.
Now I string the crosses just like I would if the mains had just been done. I first check the mains and if I find any notching, I use the side where the notching is less for the crosses to rub/contact this time. Slick poly does not notch nearly as bad as full gut or multi.
When the racquet is unclamped, the tensions on the clamps are just like they were when I mounted the racquet.
I have done this a number of times and have not found a downside yet.
Hope this helps,