slackpkg upgrade-all and chroot

Hello all !
First of all, I've searched trough forum with no luck ( not good at search here ).
I was considering to upgrade my stable current ( from 15.0+ ) to recent one but I'm scared to mess everything up, so I thought I could take advantage of having enough partition ( and space ) on my disk to replicate my actually installation on another partition and upgrade there, to have a very fast way to rollback, just in case ( by bootng from one partition or the other ).
I' ve then dump&restore my actual root :

Code:mount /dev/nvme0n1p8 /mnt/2
cd /mnt/2
dump -0f / | restore -rf -
df -h | egrep "root|mnt"
/dev/root 54G 43G 8.1G 85% /
/dev/nvme0n1p8 54G 43G 8.2G 84% /mnt/2So I'm ready to begin.

I then do chroot /mnt/2 but all tmpfs are missing ( including /dev /sys and so on ):
Code:darkstar root ~# chroot /mnt/2
darkstar root /# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/nvme0n1p7 54G 43G 8.2G 84% /Should I "mount -o bind" them or I can manage to complete the operation without them ?
Would, eventually a
Code:ROOT=/mnt/2 slackpkg upgrade-allwork, instead of a chroot ?

For those who asks why I'm not simply upgrading, now that I have a backup, the idea is that I can continue use the machine and not need a telinit 1 ( as suggested )

Thanks in advance for every suggestion.

Pierluigi

Thanks in advance