Issue with Storage Engine
I have not had much experience switching the myisam_data_pointer_size option from 4 to 6.

Simply put, I think you got it right based on the Documentation. Here is why:

According to the MySQL Documentation 5.5 on myisam_data_pointer_size:


  The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6. See Section C.5.2.12, “The table is full”.


The fact that you changed it from 4 shows that the table was created originally in an earlier version of MySQL (default pointer size is 4 in MySQL 4.1).

Looking at the additional MySQL 4.1 reference for “The table is full”:


  You are using a MyISAM table and the space required for the table
  exceeds what is permitted by the internal pointer size. MyISAM creates
  tables to permit up to 4GB by default, but this limit can be changed
  up to the maximum permissible size of 65,536TB (2567 – 1 bytes).
  
  If you need a MyISAM table that is larger than the default limit and
  your operating system supports large files, the CREATE TABLE statement
  supports AVG_ROW_LENGTH and MAX_ROWS options. See Section 12.1.5,
  “CREATE TABLE Syntax”. The server uses these options to determine how
  large a table to permit.
  
  If the pointer size is too small for an existing table, you can change
  the options with ALTER TABLE to increase a table's maximum permissible
  size. See Section 12.1.2, “ALTER TABLE Syntax”.


Since you ran the change and MySQL did not keel over in agony, you must have gotten it right.

CAVEAT #1

I noticed the table characteristics you posted


Rows: 1786679 (that's 1,786,679 rows)
Avg_row_length: 2403
Row_format: Dynamic


What disturbs me is the average length. Why? I have easily seen 1 Billion Row MyISAM tables and never had a client switch its pointer size. This may be a good time to reevaluate the table's layout.

Here are some possible ways to shrink the table without touching the pointer size ?


Does the table have a lot of TEXT/BLOB fields ?
Could those TEXT/BLOB field be removed an placed in another table ?
Could the table stand some normalization ?
Do you have INT fields that could be shrunk to TINY, SMALLINT, or MEDIUMINT ?


CAVEAT #2

The next time you get table is full, please do not automatically assume that the table reached its internal limit. Always check the datadir to make sure the disk itself is not full.