The fact that a personal name is stored in EBCDIC is missing the point. the mainframe has supported unicode for 20 years. The bank account holders name is stored in a character column in a database. Every character column can be defined with a different codepage, so it is possible to define the name column as stored in UTF-8, EBCDIC or any of a number of other codepages.

For the bank to claim that the reason that they can store names containing diacritics because the name is coded in EBCDIC is nonsense. The bank just needs to change the database column definition to UTF-8.