When converting a cash or bank account to the other type, contents of custom fields do not carry over to identically labelled custom fields in the new account. For example, identical custom fields are set up for bank and cash accounts:
Content is entered is the custom field for a bank account:
The bank account is converted to a cash account, but the field content does not carry over:
However, when the cash account is converted back to a bank account, the custom field content is restored. So it is being preserved, but not displayed. In fact, if content of the custom field is edited while the account is a cash account, the edited content is not carried back when reconverting. But the original field content, entered while the account was first a bank account, is displayed.