Good news concerning Migration from W10 1709 to 1803. The quiz question with Bitlocker enabled client Migration has now been solved.
You can direct in setup.exe of the W10 1803 handle options. You can even try to migrate with Bitlocker enabled IF it fails it will redo with Bitlocker halted (suspended or paused) or turned off.
https://docs.microsoft.com/en-us/windows/whats-new/whats-new-windows-10-version-1803
https://techcommunity.microsoft.com/t5/Windows-Insider-Program/Windows-Defender-System-Guard-Making-a-leap-forward-in-platform/m-p/167303
https://blogs.technet.microsoft.com/mniehaus/2018/05/02/new-upgrade-to-windows-10-1803-without-suspending-bitlocker/
DISM /Online /Initiate-OSUninstall
– Initiates a OS uninstall to take the computer back to the previous installation of windows.
DISM /Online /Remove-OSUninstall
– Removes the OS uninstall capability from the computer.
DISM /Online /Get-OSUninstallWindow
– Displays the number of days after upgrade during which uninstall can be performed.
DISM /Online /Set-OSUninstallWindow
– Sets the number of days after upgrade during which uninstall can be performed.
Setup.exe /BitLocker AlwaysSuspend
– Always suspend bitlocker during upgrade.
Setup.exe /BitLocker TryKeepActive
– Enable upgrade without suspending bitlocker but if upgrade, does not work then suspend bitlocker and complete the upgrade.
Setup.exe /BitLocker ForceKeepActive
– Enable upgrade without suspending bitlocker, but if upgrade does not work, fail the upgrade.
W10 1803, WIN0 1803 Enterprise customers