Let $N\ge2$ be a fixed positive integer. There are $2N$ people, numbered $1,2,...,2N$, participating in a tennis tournament.  For any two positive integers $i,j$ with $1\le i<j\le 2N$, player $i$ has a higher skill level than player $j$. Prior to the first round, the players are paired arbitrarily and each pair is assigned a unique court among $N$ courts, numbered $1,2,...,N$.  

During a round, each player plays against the other person assigned to his court (so that exactly one match takes place per court), and the player with higher skill wins the match (in other words, there are no upsets).  Afterwards, for $i=2,3,...,N$, the winner of court $i$ moves to court $i-1$ and the loser of court $i$ stays on court $i$; however, the winner of court 1 stays on court 1 and the loser of court 1 moves to court $N$.

Find all positive integers $M$ such that, regardless of the initial pairing, the players $2, 3, \ldots, N+1$ all change courts immediately after the $M$th round.

[i]Proposed by Ray Li[/i]
To solve this problem, we need to determine the number of rounds \( M \) such that players \( 2, 3, \ldots, N+1 \) all change courts immediately after the \( M \)-th round, regardless of the initial pairing. 

1. **Initial Setup and Definitions**:
   - There are \( 2N \) players, numbered \( 1, 2, \ldots, 2N \).
   - Player \( i \) has a higher skill level than player \( j \) if \( i < j \).
   - Players are paired arbitrarily and assigned to \( N \) courts.
   - During each round, the higher-skilled player wins and the players move according to the rules specified.

2. **Movement Rules**:
   - Winner of court \( i \) (for \( i = 2, 3, \ldots, N \)) moves to court \( i-1 \).
   - Loser of court \( i \) stays on court \( i \).
   - Winner of court 1 stays on court 1.
   - Loser of court 1 moves to court \( N \).

3. **Objective**:
   - Find \( M \) such that players \( 2, 3, \ldots, N+1 \) all change courts immediately after the \( M \)-th round.

4. **Key Observations**:
   - Player 1 (the highest-skilled player) will eventually reach court 1 and stay there.
   - We need to ensure that players \( 2, 3, \ldots, N+1 \) change courts after \( M \) rounds.

5. **Analysis**:
   - Consider the initial configuration where players are paired arbitrarily.
   - After each round, the players move according to the rules.
   - We need to track the positions of players \( 2, 3, \ldots, N+1 \).

6. **Tracking Player Movements**:
   - Let’s denote the positions of players \( 2, 3, \ldots, N+1 \) after each round.
   - We need to ensure that these players change courts after \( M \) rounds.

7. **Constructing a Solution**:
   - We can use a combinatorial approach to determine the number of rounds required.
   - We need to ensure that the players \( 2, 3, \ldots, N+1 \) are in different courts after \( M \) rounds.

8. **General Case**:
   - For \( N \geq 3 \), it can be shown that \( M \geq N+1 \) rounds are required for the players \( 2, 3, \ldots, N+1 \) to change courts.
   - This is because the players need to cycle through the courts, and it takes at least \( N+1 \) rounds for this to happen.

9. **Special Case for \( N = 2 \)**:
   - For \( N = 2 \), it can be verified by hand that \( M = 2 \) rounds are sufficient.

10. **Conclusion**:
   - For \( N \geq 3 \), the minimum number of rounds \( M \) such that players \( 2, 3, \ldots, N+1 \) change courts is \( M = N+1 \).

The final answer is \( \boxed{ M = N+1 } \) for \( N \geq 3 \) and \( M = 2 \) for \( N = 2 \).