Add a formula field:
SUBSTITUTE({Activities (unformatted)}, "|", ",")
This will replace all vertical bars with commas. Now copy the entire contents of this field into your {Activities (formatted)} field, and Airtable will convert the comma-separated items into multiple-select entries record by record. Any duplicate entries will merge with existing ones.
The only trouble you’re going to run into is with the records that contain commas as part of someone’s response (i.e. the “Other” options), like record 101:
One way around this is to use this for the formula instead:
'"' & SUBSTITUTE({Activities (unformatted)}, "|", '","') & '"'
This will ensure that each entry gets wrapped in quotes. When copied and pasted into the multi-select field, those entries with commas will retain their commas, instead of the commas separating them into further choices. You’ll still have to deal with a bunch of “Other” items in your multi-select list, but entries like the example above will create a single item—"Other: sail, ski, paddleboard"—instead of three items—"Other: sail", " ski", " paddleboard".
