text columns can only be converted to:
nchar
varchar
image columns can only be converted to:
varbinary
binary
You cannot modify char or varchar datatype columns to text columns. If you are converting from text to char or varchar, the maximum length of the column is governed by page size. If you do not specify a column length, alter table uses the default length of one byte. If you are modifying a multibyte character text or image column, and you do not specify a column length that is sufficient to contain the data, Adaptive Server truncates the data to fit the column length.
