I would suggest you to follow below VMware KB.
Export dvSwitch configuration from 5.5 and import in vCenter 6.
Create standard switch on ESXi Host in vCenter 5.5
Migrate all VMs and vmkernel to standard switch ( do not remove all pNICs from dvswitch or you will loose Host and vm connections )
Remove ESXi host from dvswitch
Disconnect ESXi host from vCenter 5.5 and Add Host to vCenter 6. ( disable HA before you remove Hosts from 5.5 )
Add Host to dvswitch and migrate VMs from standard switch to dvSwitch.
See below URL, if you want to migrate Host with dvSwitch and without migrating VMs to standard switch. But this is not recommended in production setup.