Isto é o que eu normalmente uso, embora um pouco manual:
/* Add a 'temporary' UDDT with the new definition */
exec sp_addtype t_myudt_tmp, 'numeric(18,5)', NULL
/* Build a command to alter all the existing columns - cut and
** paste the output, then run it */
select 'alter table dbo.' + TABLE_NAME +
' alter column ' + COLUMN_NAME + ' t_myudt_tmp'
from INFORMATION_SCHEMA.COLUMNS
where DOMAIN_NAME = 't_myudt'
/* Remove the old UDDT */
exec sp_droptype t_mydut
/* Rename the 'temporary' UDDT to the correct name */
exec sp_rename 't_myudt_tmp', 't_myudt', 'USERDATATYPE'
