Hello.
The software downgrade for the OLT5683T can be done from V800R016C00 to V800R015C00.
For the downgrade, first you make sure the backup is done successfully.
Also, you should be aware that the downgrade is not an easy operation.
For more details, you can consult our support site for the upgrade/downgrade guide.
http://support.huawei.com/enterprise/en/index.htmlThank you!