I receive an error message of Invalid Parameter: conversion value when attempting to change the format type of custom field on an existing custom form.
When this message is received, more often than not there is an existing custom field value that isn't the value type that you are attempting to change the field format to. Example: you are attempting to change a field that is listed as the Text format to the Numeric format and you receive this error message. In this example it was found that most of the field values were numeric but there was one that was entered in as a text value. This single text value was throwing the Invalid Parameter: conversion value message. Reviewing all the values entered in for the custom field using a report will point you to the field value that is causing the "Invalid Parameter: conversion value. Once the value is found it can be addressed so that Format change can be applied system wide.