Unable to update recurring amount and number of installments

I first mentioned this in issue #21 (closed), but am making a separate ticket about it now.

Attempting to update the billing amount/number of installments for a recurring contribution results in the UI alert:

Authorize.net (Credit Card) E00013: Start Date cannot be changed.

There are no other related civi log errors aside from this, but there is this Drupal log error:

Notice: Undefined variable: errorCode in CRM_Core_Payment_AuthorizeNetCommon->changeSubscriptionAmount() (line 609 of /var/www/web/public/civi/extensions/authnet/CRM/Core/Payment/AuthorizeNetCommon.php).

Is the recurring series start date getting changed when this is sent, rather than grabbing the original start date from the record (or not attempting to send an updated start date at all)?

I have not looked at the code for this yet to see what all is being sent for this, but wanted to at least make sure the issue was noted.