You can schedule this update.

Just comment out \Elgg\Upgrades\ContentOwnerSubscriptions::class, string in this file https://github.com/Elgg/Elgg/blob/5.1/engine/upgrades.php

Like Nikolai said. You can do this upgrade later.

Once you've migrated your site to 5.x you can re-enable the upgrade and run that specific upgrade in the CLI (check the params for elgg-cli upgrade). I don't know if you could run it in batches (by canceling the upgrade mid run) but most of the time these long running upgrades don't have a heavy impact on the database (you could test this). So you can have it running on the background of your live site.

Excellent -- thank you both.  This will really help simplify our migration plan.

Please use the full path to your data directory, e.g.:

/www/domainame.co.uk/G/data/

Watch a process on https://www.youtube.com/watch?v=1kY_U3USWX0

I have tried using the path to my Data directory which is > /www/DATA

This is not working also. The error i get is this > Your data directory /www/DATA does not exist.

1. The characters are case sensitive.

If your folder is named "data" then you should enter "data" but not "DATA".

2. Check the permissions on this directory.

https://learn.elgg.org/en/stable/intro/install.html#create-a-data-folder

Already discussed here (read my replies)