Tricky questions. The usual process to process the upgrade is:
1. Logon to the Singapore Airlines website with your KF number.
2. Retrieve your booking. Select Upgrade from the menu options and then process accordingly.
With the recent changes to the SQ website I can't assist. I'd expect the option to be available on the screen when you vies an existing booking.
As for the 'Promotion' fare being upgradeable it would depend on the terms and conditions of the fare.
In light of the current website issues and the uncertainty of the fare applicability I think you are going to have to call the Krisflyer centre on 02 9350 0203.
Prior to the call, consider doing some searches on the SQ website to make sure that redemption seats are available on your flights.
If your fare is valid for the upgrade, please be sure to plead that you couldn't complete the booking on line and therefore should still get the 15% online discount for the upgrade processing.
Good luck, please report back on what happened.