To change a user
defined data type, all tables using it must be dropped, the data
type must be dropped and everything has to be recreated.
system table contains the names of all the User Defined Data Type.
Update the xtype (Data Type), length (data length), xprec (data
field precision), xscale (data field scale) and etc. according
to new requirement for that particular row which is the definition
for the data type.
Related Link: How to alter a User Defined Data Type?