Our present OpenBSD server needed more disk space, so we replaced the 13 GB disk (it's pretty old too) with 150GB. An image was made and then restored on the new disk. So far, so good. The new disk has a 13GB OpenBSD partition, it boots and works as before.
Next order of business: use the other 135 GB. With fdisk I can see it's there and I can change the file type (originally Extended LBA and it's back to that). But using df it doesn't show the new space. I had a look at fdisk and disklabel but I can't figure out what to do to make the space available.