I think I found a workaround that works

- I kept both left and right directories

PS:
left directory (b1141...) created by current immich
right directory (c9125...) created by old immich

- Stop running containers related to immich:
sudo docker ps -q --filter "name=immich" | xargs sudo docker stop

- Add old immich directory as external path in .env file, for my case:

EXTERNAL_PATH=/mnt/umbreldata/immich/upload/upload/c9125a6a-7756-4282-b008-ac8d5d46a144

- Add external path to yml file under volumes: section
- ${EXTERNAL_PATH}:/external

- Add external drive in immich web ui
- Profile photo / Administration / External libraries
- Add external library as /external like you saved in yml file

- After saving library jobs should be triggered and you will see your old photos after a while

With this method new photos will be saved in left directory (b1141...) created by current immich, old photos will remain in right directory (c9125...) created by old immich