Thanks for the info on your app, Daniel. I’ve downloaded it and it has successfully bulk edited the metafields as I needed.
For reference for other users:
In the Ablestar app you can filter down all of your products by their existing ‘tag’, and then run a bulk update to apply a specific ‘metafield’ value to those products. You can only run 5 different updates per 30 days on the free plan. So if you had 50 tags to copy over to metafields it would take 10 months on the free plan! (or pay for the app to get unlimited edits)
Another solution -
Since the creation of this original question, Shopify has added a new option that simply displays all of your old tags within a filter in the store 2.0 metafield filters…
Go to Search & Discovery app > Filters > Edit Filters and click the “Tags (displayed as More filters)” option. This is not necessarily as flexible for the end-user as defining info in new filters properly but it does get the old Tag filters displayed on the new system in a couple of clicks.