How to change text to longtext.

I want to store more than 64kb of data in Case->Description field.

The solutions I have tried so far:
modifying DB type for description field to 'longtext' - this did not survived QRR.
modifying field_metadata table in db to 'longtext' - this too did not survived QRR.

modifying custom/Extension/modules/Cases/Ext/vardefs - This survive the QRR but change UI field from text-area to solid text field.

We want to change the type of the description field from 'text' to 'longtext' and keep the UI as text-area in sugar 8.0 professional ? Any help will be appreciated.

Thanks