Give the sand a good backwash and rinse. I suspect the filter is very clean since the sand is new and you did not have a huge load of solids filtered by it. The media change is required during the standard conversion process because the baqua-goop is so thick and sticky that it just fouls up the media. You should be good.
Add the sock with the 15ppm CYA and give it time to circulate and dissolve. Like I said, knead it’s occasionally to help it dissolve and give the pool 24 hours to mix. Brushing around and granules that fall to the surface helps. After you place the sock, get the chlorine up to 15ppm and home it there as best you can. It’s going to take time for the CYA to dissolve so you’ve got a couple of days of FC/CC testing and chlorine dosing ahead of you. Wait until the CYA is all gone before you test for it.
My guess is that you’re in the home stretch now. Good luck!