The interchange of two rashi lords forms a special planetary combination called parivartana yoga.
There are three kinds of parivartana yogas according to nature of involved houses.
A) Maha Parivartana Yoga: when both the interchanged lords belong to kendara, kona, dhana or labhshatna. In other words, there should be mutual interchange between lords of 1, 2, 4, 5, 7, 9, 10 or 11 houses. For example, lord of 1st house is in 5th and lord of 5th house is in 1st; or lord of 9th is in 10th and lord of 10th is in 9th. There are 28 possible maha yogas.
B) Kahala Parivartana Yoga: when there is interchange of sign between 3rd lord and 1, 2, 4, 5, 7, 9, 10 or 11 house lord. For example, lord of 3rd in 10th and lord of 10th in 3rd house. There are 8 possible kahala yogas.
C) Dainya Parivarana Yoga: when one of the involved lords is of dushtana bhavas (6, 8, 12) and other one belongs to 1, 2, 3, 4, 5, 7, 9, 10 or 11 houses. For example, lord of 6th house is in 11th and lord of 11th house is in 6th; or lord of 8th is in 10th and lord of 10th is in 8th. There are 30 possible dainya yogas (if 6 kind of parivartana among dushatana lords that leads to Vipreet Yoga, are excluded then there are 24 such possibilities).
Following are the effects aforementioned three kinds of parivartana yoga in the maha/antar dasas of involved planets.
A Person born with Mahayoga will have divine blessing and will be adorned with expensive and beautiful clothes and ornaments. He is revered by the king (honoured by authorities). He will hold a high position and will be rewarded of authority by the king, He will be blessed with sons and fully enjoy wealth and conveyances. (Phaladeepika)
The native with Kahala Yoga will be capricious and occasionally be haughtly and sometimes sweet in his speech. There will be occasions when he will be very prosperous and then will be driven to poverty, unhappiness and misery. (Phaladeepika)
The person with Dainya Yoga at birth will be a fool, will revile others and indulge in sinful deeds. He is always in trouble from his enemies. He will speak harshly and will not have a stable mind. He will encounter obstacles in all his ventures. (Phaladeepika)
If two planets say A and B are in parivartana. Then in the dasa of planet A, result of the B’s occupation (as lord of the house tenanted by A) shall obtain. Vise versa holds true for dasa of planet B.
Parivartana is the strongest relation between any two house lords. In panch-sambhandh classification, the interchange of house lords is known as kshetra sambandh and allotted full power of 60 virupas. If the concerned lords own good houses, expected result will be auspicious, whereas in case of interchange between good and bad (6, 8, 12) houses, evil result will manifest. Like rashi chakra (D1 chart), parivartana may also form in any of vargas (divisional charts) but the exchange in varga shall be limited to the significations of that divisional chart. There are some schools of thought who hold that parivartana occurs when planets mutually exchange lunar constellations (nakhatras) but noted classical works do not support this thesis.
A Point to Note
Except Sun and Moon, rest of five planets (Mercury, Venus, Mars, Jupiter and Saturn) own two signs. When parivartana occurs between any two (good or bad) signs, the involved planets do not yield result of their second sign. Take an example of Leo ascendant where Mars is in Sagittarius (5 house) and Jupiter is in Aries (9th house) then neither Mars will give the result of Scorpio (4th house) lordship, nor Jupiter will yield result of Pisces (8th house) ownership. This exchange should only analyse as interchange between 5th and 9th lords. Similarly if there is a parivartana between Sun and Saturn for Gemini lagna then it will only going to yield kahala parivartana yoga between lords of 3rd and 9th houses.
Keeping other factors constant, suppose for a Cancer langa, there is a parivartana between 8th lord Saturn and 9th lord Jupiter. That is, Jupiter is in Aquarius and Saturn is in Pisces. According to Parivartana yoga classification, this is an evil planetary signature (dainya yoga). Now the question is what type of result may manifest in the dasas of Saturn and Jupiter.
The answer is both the dasas of Saturn and Jupiter shall yield inauspicious results. The dasa of Saturn shall give result of its dispositor Jupiter. In other words, during dasa of Saturn, one shall experience effect of 9th lord (Jupiter) in 8th house (Aquarius) which is indeed bad. Similarly, the dasa of Jupiter shall yield result of Saturn. That is, in the dasa of Jupiter, native shall accrue effects akin to 8th lord (Saturn) in 9th house (Pisces). However, the mutual maha and antar dasas (Sat-Jup and Jup-Sat) will surely give pronounced evil effects of this parivartana yoga between 8th and 9th houses. Also note that Jupiter is unable to produce any good result of Vipreet (6th lord in 8th) because of the bondage of 8th and 9th house exchange.
Here for Aquarius lagna, Venus and Jupiter are exchanging 4th and 2nd lordship. This parivartana leads to maha yoga and surely benefit the native with dhan and sukh when dasa/antardasa of Venus and Jupiter operates.
In the dasa of 4th lord Venus, one shall enjoy the result of 2nd lord Jupiter in 4th house. Similarly in the dasa of 2nd lord Jupiter, one shall enjoy the result of 4th lord Venus in 2nd house. But in mutual antardasas of Ven-Jup and Jup-Ven, native will definitely earn amass of wealth, receive higher learning, command luxurious vehicle and obtain pleasure of domestic comfort and sumptuous meal.
Suppose Mars is in 5th house (Libra) and Venus is in 6th house (Scorpio) for the following (hypothetical) Gemini ascendant.
Now in the dasa of Venus, one shall come across result identical to its dispositor i.e. 6th lord (Mars) is in 5th house (Libra). This may include differences and disputes with lover or off spring, loss in speculation, sickness of children etc. Similarly in the dasa of Mars, native shall face evil circumstances as if 5th lord (Venus) is in 6th house (Scorpio). As a result native’s own children turn hostile to him, he may get separation from them, fear of abortion, confrontation with officials etc.