altering tables and constraints

I'm in the process of trying to convert a database such that all the strings
(VARCHAR) are converted to wide strings (NVARCHAR). I have a script that
accomplishes this by removing all the primary key constraints, converts the
necessary columns, and then replaces the constraints. The script walks the
sysnames table and stores all the constraints in a table variable, and
constructs a script to recreate all the constraings based on the 'xtype'
column from sysindexes (this is based on the system stored procedure
sp_pkeys). The script creates a constraint if the xtype is of type 'PK', or
creates an index based on the INDEXPROPERTY of the index, whether it be
unique, and either clustered or non-clustered.
This works for the most part, but I have found that there are constraints
being created on some columns that did not exist before the conversion. For
example, I have a table which has a primary key on it's identity columns
defined to automatically insert a new value at each insert incremented by 1.
After the conversion, there is an additional constraint placed on this table
which prevents a value of NULL from being added, which should be a problem
due to the IDENTITY column, yet attempting to do an insert on this table
generates an error saying that a NULL value cannot be inserted. I'm not
manually inserting anything, this should just bump the id value by one and
do the insert, but this new constraint prevents this, leaving me with a
table I can no longer insert into.
In another case, I have several varchar columns that have default
constraints (simple text strings), which are also dropped before conversion.
Upon replacing the constraints read from sysnames, I get similar errors
regarding not being able to insert nulls into these columns, which I didn't
get before, as these columns had default values.
My questions are, is it possible to exactly recreate constraints
programmatically? Is there a preffered method for converting databases from
narrow to wide character?
Thanks for any advice,
-Gary
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