You can create a new single line text metafield with one value and updating those.
The process would be:
Download the product metafields data into excel through an app like Matrixify.
Find the old metafield you wnat to amend. The values in the sheet would be in a format like this:
[“List Value 1”, “List Value 2”, “List Value 3”]
3) Create a new metafield on Shopify with only one value instead of list option. Add the new metadield in the sheet
To merge the list values into one single line, you can select the old column and use the Find+Replace function to remove “” and
The updated values should look like Value 1, Value 2, Value 3
Upload the product CSV again and now you would have a new metafield with all your required values. You can delete the old one or keep it.