normally parivartan is exchange of rashi Lords e.g. Venus in pisces and jupiter in libra
Does parivartan take place for natural karak houses?
e.g. Mercury in 3rd House (Mars karak) & Mars in 6th house (Mercury karak)
Or sun in 4th House moon in 5th house
No, parvirartan is exclusive to rashis. Karakas are only used when analyzing houses.