Nute build-up, not salt build-up. The MG has time-release ferts in it already, so in effect, they are getting double the nutrients.

Would lay-off the additional nutes for now, and watch their reaction. Do you use micros or other additives?

Transplanting at the begining or early stages of flower, (within around 1 or 2 weeks after switching to 12/12) is ok, but the deeper you go into flower, the more actual control of the nutrients you'll need to have. (MG nutes should be depleted by week 4 or 5, latest) I used to always up-pot when I switched to 12/12, and they were fine. The sativa dom's I work with need less nutes, so I would wait a tad longer before starting the nutrient regimin on them. The plants will tell you what works, and what doesn't.

This is a basic way to handle the MG soil. You'll need to make little adjustments, but if you have any problems, or something doesn't look right, just holler. Would seek guidance from those that use the BioBizz stuff, tho. I'm a Fox Farms guy.