Hi Yogi,
I looked into this issue and I have been able to replicate it. This issue appears to be a bug because I'm able to create fields in a file geodatabase with 64 characters, as per the documentation, but the Alter Field management tool appears to be restricted to 31 characters.
Defining fields in tables
http://desktop.arcgis.com/en/desktop/latest/manage-data/geodatabases/design-defining-fields-in-table...
If you have an active support account could you send me a private message with your account information so that can attach your information to the bug I'll log for this issue?
Workaround
At the moment the only workaround I can see for this issue is to use the Add Field tool. Unlike the Alter Field tool, this tool will modify the order of your fields. If field order is important to you I would suggest either using python to create a new template feature class with your updated field and loading your data into this new dataset prior to overwrite the original class or create a fieldmappings object against your data, updating the information for the needed field, and then using the Feature Class to Feature Class to create a new clone of your data prior to overwriting it.