I use an approach to upgrading that I have not seen written anywhere. It does need additional space in the root partition but with disks these days that is not normally a problem.
First copy away the "important parts" of the root partition onto another partition. What is the "important parts" is up to you, but should include any changes that you have made to the root partition. I use the ROOTBACKUP code in /etc/daily which copies the root partition to another partition on a separate disk, so that is done automatically. Now do an install of OpenBSD, but using only one partition (plus the swap partition). Do not change your disk partitioning in any manner, Do not use the install to mount any other partitions. All of OpenBSD will now reside in one partition. Which is why you need more disk space in the root partition. Note that all the other partitions will be untouched by the install. Once the new system comes up, you can now mount the partition that you saved away your "important parts", If you saved them previous under say /home mount what was /home under a different name. Merge your changes to "/etc" and others into the new root partition. Mount each of the old partitions that would have contained information set by an install (such as /usr) under a different name. Merge the new files from what is now in the root partition just installed into the correct place in the old partition. Merging what was in the old /etc into the new /etc, will of course update /etc/fstab which when the system is booted will end up mounting the old partitions into their correct places. Boot the system. The updated /etc/fstab will now mount the partitions in their correct places. The /usr and /home and other from the install are now inaccessible and have been replaced by ones defined in /etc/fstab. Packages can be installed at any time during this process, you just have to make sure that if you add packages while you are operation in the one partition that files added by the packages are also copied to the right place in the old partitions.
