htc u ultra how to downgrade from 8 to 7 ?

hello supporter

htc u ultra bootloader is unlocked
info:

Code:

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Reading information...
Using Fastboot mode...
Reading common information
(bootloader) kernel: lk
(bootloader) product: htc_ocedugl
(bootloader) version: 1.0
(bootloader) max-download-size: 1578400000
(bootloader) serialno: FA7C11700414
(bootloader) slot-count: 0
(bootloader) current-slot:
(bootloader) imei: 355167083572220
(bootloader) version-main: 2.19.401.1
(bootloader) boot-mode: download
(bootloader) version-baseband: 2.0.U010051c@80211.2
(bootloader) version-bootloader: 1.0.0.0000
(bootloader) mid: 2PZF20000
(bootloader) cid: HTC__J15

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is there any way to downgrade from oreo to nougat?
i try to reset sw by power adpter but with this error :
Starting PowerAdapter procedure:
Detected model: (Android 8.x)
Sorry, this model is not yet supported. If you think this is an error, please contact support