Text list has too many characters

We have an application written in 4.6, which has been designed to hold values in text lists. These values are postal codes. The business has changed the way it behaves and has added virtually all the postal codes in the country into one of these fields. We now have errors when trying to save the documents due to the amount of characters in the field. Apart from re-assembling the functionality, is there a workaround?
It's not clear to me whether the document that contains the long list of postal codes is a regular data document, or a reference document that contains the valid values for a keyword field, accessed via @DBLookup.

If the document is a data document, there's not a lot you can do, apart from changing the multi-value text field to a Rich Text field. That will hold all the data you want, but the values will not be available in views and agents. Breaking the field up into multiple multi-value text fields will probably not do any good, because the limit of summary data applies to the document as a whole, not just to each field.

If the document is a reference document, you can break it into two or more documents, each containing a subset of the postal codes and the lookup will still work, provided you don't exceed the limit of 64,000 bytes returned by @DBLookup or @DBColumn.