I would bias towards the player's new team based on number of players from the old team. Say Team A decided to leave its current sponsor of Team A. Their new name Team B will be replacing "Team A" in the rankings unless stated otherwise by an outside settlement which will need to be communicated via pm to me.

But if negotiations can be conducted for both sides to ensure a better settlement. The old team might just be left there for sometime and both parties start afresh with new team names if a settlement cant be agreed. However, after a settlement is made, the new team with its new roster will be assumed from the settlement date onwards. In other words, future "roster" of the team will be based on the new roster from the settlement period onwards.

The ranking system serves as a guideline for seedings to be done by the organisers. I highly doubt organisers will follow the ranking system 100% in their seedings based on my past experiences.

Referring back to your example, as an organiser of this 20k tournament, I would seed Team XYZ in place of the "former team ABC" as the top ranked team in Malaysia in my brackets due simply to the fact that they have 4 of the last known members of "team ABC" in the rankings. The rankings only will be updated after the tournament is over, as such seedings in important tournament like this should be done with in depth knowledge of the scene in which many here would be ready to share if asked politely.
As the ranking maker, the teams will have the responsibility to approach me via pm for a settlement on the issue with me heavily biased replacing "team XYZ" to "team ABC" in the rankings due to the fact they had more than 3 of their former members in the team. Unless both sides agree to the current status quo with "team XYZ " starting afresh, the default situation will be to replace "team ABC" to "team XYZ" in the rankings and "team ABC starting afresh in the rankings.

The Malaysian Dota Scene is as vibrant as the international one with team loyalties can be bought or traded with the turn of the thought or the start of a petty argument over a single solitary pub-game. The default solution will be the "3 player rule" of the last participating tournament team roster that will be applied. Issues will arise when the "3 player rule" cant be applied such as a split of 2-2 and 5th player retiring. If this happens, the teams will have the responsibility to contact me of a settlement for the ranking system or both teams that start with new names will be started afresh in the rankings while their old team name will be "freezed" in the rankings until former team members of the team decides to reactivate it with the blessings of 3 of the last 5 tournament participating players of that team bearing that name.

The rankings is manually done on an excel sheet in which it will be looked through by independent team managers or players that requested it via pm with their email attached. The forum system did not allow me to upload an Excel file.

The UC_PRO case has been resolved with UC_PRO Tbun w4si claiming all the points at SMM 2008 with their new name UC_PRO in the rankings with blessings from w4si founder and manager Lucifer.

The full list of tournaments counted in this ranking to date are :

1. SGNDT last 16
2. QUAS
3. Borneo
4. Invasion

I fully understood the risk of making a ranking system in a subjective team game of Dota. That is why I try to involve as many people as I can in making it along the way. This project boils down to be my way to see which teams are the best in Malaysia and hopefully will boost participation of tournaments by encouraging teams to fight for a higher rank in a tangible predictable ranking system.

Well the problem with players jumping from team to team can be solved by dividing those points among all the 5 players.
What underminer is trying to do is to protect sponsors from losing all the points they built up for their home team, but as far as i know its really difficult to keep players.

Therefore, if you want an accurate point system, the points should be divided by all 5 OR 6 members of the team. However, your excel file will be huge In the end we might even get points for 3v3 or 1v1 and we will be able to know who is the TOP dota player in Malaysia 

Aiyo so mahfan. If team-based calculations are messed up due to ever-changing player roster, then just implement an individual point system. Every single game, if a player is in a winning team and achieved something, setup a milestones of comparison and distribute the points evenly. Then, a team consists of any Tom, d*** and Harry will able to be seeded accordingly to their personal achieved points.