backup, reparation, format, restore
https://forums.freebsd.org/showthread.php?t=185.
I suppose you have virtual disk image. If i was you, i would make another disk image (with more space if necessary), portion it, make labels, format. and then simply use dump/restore
let's say you want to dump /dev/ad1s1a to new /dev/ad2s1a
mount new slice to some path, cd to it and
dump -0Lauf - /dev/ad1s1a | restore -rf -
this will basically move files from one disk to another.
you might need some /tmp space
you will need to repeat this for all partitions
there are also other options.
but using this you will have 2 virtual disks with same data.
If anything fails you still got original disk, if all works well (as it should), remove disk 1 and keep using disk 2.
if you can boot etc.... you car finally delete original disk
i suggest dump/restore because that will ensure you keep owner/group permission.