Hi,
Yes unfortunately I must advise against this kind of change. This is a "system" attribute which means it is controlled by Service Desk You can tell which attributes are system ones as their "Name" property does NOT start with an underscore.
The next time you try to upgrade this database Service Desk will try to resize this attribute back to its "proper" length and this will likely fail now. I strongly recommend you restore your database to before you made this change to avoid further issues and leaving your database in an unsupported state.
In regards to your original question - the best answer is to create a new attribute and ignore the existing one. If you want to copy existing data across you can do this in an UPDATE statement in the database, ie. UPDATE table_name SET new_col = old_col.
Thanks
Stu