A quick sweep of the forum hasn't thrown up an answer to this for me:
I have just got my HE. Before, i had hue bulbs, and tradfri bulbs added to the hue hub. I had the gateways for both and used Homekit for control (no automation, just manual on/off/dim and colour).
I added the Hue hub to HE and all the bulbs are there. Now though, the Tradfri bulbs report in the Hue app as 'unreachable'. You can switch them etc in HE but nothing actually happens. If i dim in HE with the hue app open, the control is mirrored to the Hue app, but nothing happens. I have moved the three hubs for testing purposes to eliminate range as a problem (there are about 5m away with clear line of sight).
I can correctly control the Hue bulbs via HE (success!) and both the Tradfri and Hue will still work via Homekit. The tradfri will still work via the Ikea Homesmart app.
I tried unplugging the Tradfir gateway but that didnt help re the Hue app/HE situation.
These two statements are seemingly incongruent, because a zigbee device can only be paired to a single zigbee coordinator, i.e. either the Hue bridge or the Tradfri gateway.
Were the Tradfri bulbs directly paired to the Hue bridge? Or does Hue have a Tradfri gateway integration?
You know something - i have never really looked at it that way until now. Having used Homekit for so long i havent dipped into the Hue app for some time. Maybe its been like that for a while (ie saying the ikea bulbs are unreachable).
So my answer would be to unpair the Tradfri bulbs from the Ikea gateway (losing HK integration in the process) and that should free them up to work with the Hue gateway and thus HE?
You can regain HK control using HomeBridge or HOOBS. You will need an RPi for HOOBS (or purchase a HOOBS appliance), while HomeBridge will run on any hardware that can run node applications and is left on permanently - an RPi or a Mac mini (for eg).