Most of my revenue bookings are done through my company corporate travel agent.

If you upgrade with miles, there is no restrictions in you getting the 15% discount for upgrade, you just need to do it online.

However, it is possible that the fare booked by your employer is not eligible for upgrade, in which case you will receive the following message: "The cabin class/fare family cannot be upgraded."

You can then ask your TA to re-issue the ticket on a higher class that can be upgraded. Only S, Y, B and E can be upgraded. You then proceed online and upgrade your ticket if there is availability.

You only lose the 15% discount if you are able to complete the process and choose to call an agent instead.

However if you are having a technical problem while trying to upgrade the ticket and cannot do so online, just state this to the call center agent and they will apply the 15% discount.