I don't think this is easily done automagically as the redirection plugin has no way of knowing what year or month to replace the category with.
It can automagically add a redirect if you change a post-title, but not if you change the permalink structure.
For the above you will need to add rules for each post, but you can group them together by month using regular expressions.
For each year/month combination you can add a rule with all the post-titles for that year/month in the source URL regex seperated by the | character. Of course you will have to replace the year and month in the target URL with the relevant info too.
Source URL: ^/subfolder/[a-z0-9_-]/(post-title1|post-title2|post-title3)[/]?$
Target URL: /year/month/$1
Hope this helps.