I'm fairly certain that the database tag format hasn't changed, so what you can do is replace dashes with spaces directly in the database. This query will do it:
UPDATE qa_posts SET tags=REPLACE(tags, '-', ' ')
Then you must do a full reindex under Admin > Stats, for Q2A to pick up the changes. If you have a lot of posts, this can take many minutes and may cause a few errors on the site while the reindex is happening.
P.S. I'd recommend doing a database backup first in case anything goes wrong.