.html at the end of my permalinks

I took over a site in which the permalink structure is /%year%/%monthnum%/%postname%.html/

I wnat to change it to /%postname%/ but when I do (with the change permalink helper activated) the urls do not redirect. I get a page not found error.

I know this may be a bit unique, just wondering if you had any ideas on what I could try?

Thanks