There is one additional point that might be worth to mention. As per other thread https://answers.sap.com/questions/12759895/system-update-does-not-change-type-of-column.html
Once the column type definition has been taken into effect in items.xml, it is not possible change the column type once it has already been created.
"redeclaring attributes" hand in hand with "item type sub-typing" functionality does work to such as, e.g. you can add a ''unique'' flag, disallow writing etc.
make the type of the attribute more specific (only subtypes can be used here)
See "Redeclaring Attributes" section for details
Only for modifying DB column type it does no work. You do need to run an alter table in DB server level.
You could do this in *items.xml definition or you could do this with direct SQL (alter...) In *items.xml you can do this with columntype element (https://wiki.hybris.com/display/release5/items.xml+-+5.1.1) . This would require update running system. Advantage with direct SQL is that you can execute it directly.