HTC has confirmed, via its support pages, that the Android 2.3 update is coming for both the HTC Desire HD and the HTC Incredible S.
The Gingerbread step-up will come courtesy of a Firmware Over The Air update, with users getting a notification on their front screens to say that it's ready.
HTC recommends using Wi-Fi to complete the update, so as to not eat up all your data allowance. When the notification pops up simply push "OK" to accept it.
When it's done, you can check that you're all up to date by referring to the build number, which should show as 2.36.405.5 for the Desire HD and 2.12.405.7 for the Incredible S.
There's no time frame on the update post from HTC, so we recommend just keeping your eyes firmly fixed to your phone's frontscreen. Try not to blink and take on fluids using a long straw.