The first assumes that your previous management company informed the board of directors about the new management company.  The management company may have been sold, changed their name, or merged with another company. 

This is not uncommon in the industry, and it is legal.  Since your agreement is with the previous company, however, your association has no obligation to stay with the new company. 

I would recommend that the board determine what kinds of changes were made before they make their final decision.  What experience does the new company have and what is their reputation in the community? 

Will the property manager stay the same?  Were there personnel changes?  It may take a few months to determine if your service is better or worse, but the wait may be worth it.

The second view assumes another board member or members made the decision and did not include you in the vote. 

The selection of a new management company typically requires a long evaluation process by the board of directors or an appointed committee.  The final decision concerning which company will be selected requires a vote of the majority of the board.

The decision as to whether or not a new management company should be hired should have been discussed at a prior meeting.  If you were unable to attend, the minutes of that meeting should reflect a motion to hire a new management company.

The selection of a new management company begins with a clear understanding of the needs of the association and may require a special meeting by the board or a committee to clarify them.

After bids have been received from qualified companies, a meeting should be held to interview the prospective management companies.  

A vote to change the management company would have been legal if there had been a quorum of the Board of directors at a properly noticed meeting called for the purpose of selecting a new management company, or if it had been on the agenda of a regularly scheduled board meeting and a majority of the directors voted for the change.

The tone of your letter suggests you were surprised by the change.  From either perspective, if it had been done properly, you should have been informed prior to the change in management.

In the executive session meetings who is entitled to attend?  Is it normal for the management company to attend even on issues that relate to the management company itself?

Executive session meetings are held for the purpose of litigation considerations, matters relating to the formation of contracts with third parties, member discipline, or personnel matters.

The board of directors and the property manager usually participate in executive session meetings.  Other attendees may include the association's attorney, other professionals, contractors or vendors, and homeowners who may be subject to a fine, penalty, or any other form of discipline.

Sometimes, the property manager may be excluded from executive session meetings, particularly if the issue being discussed involves the management company.