As per the upgrade notes:
N.B., If you require multipath, full-disk encryption, or the ability to re-using existing partitions, you will want to continue to use the alternate installer which can be downloaded from http://cdimage.ubuntu.com/releases/18.04/release/ As of 18.04.1, the Subiquity server installer now supports LVM, RAID, vlans, and bonds.
I have an existing ubuntu server 16.04. It has 2 hard drives. The boot and OS are on the first without encryption. It also has an encrypted storage drive that I manually mount.
My command line is offering to upgrade to 18.04 and I wouldn't mind just for the PHP 7.2 support (might bump that to 7.3). However the patch notes have me a bit worried. Would I still be able to manually mount my fully encrypted drive if I ran "do-release-upgrade'?
It also said I cannot reuse existing partitions. If I cannot use existing partitions what does it do to them? Merge them? Remake them?
I am just trying to get a good idea of what to expect when upgrading.