Changing formulas/breast milk frequently

Due to the chronic lack of formula in the US right now I'm having to frequently change formulas depending on whats available locally. Obviously this is far from ideal, but blending the formula during changeover period helps. I also notice my baby girl also seems to get very runny white speckled poop/diarrhea during these changeover periods too. Now i have also just received some breast milk from the surrogate, so should i also blend that or just changeover completely for a day or two? Is any one else having the same issues? Any advice please?