Permalink structure changed – broken links site-wide!

Hello – I have a major problem that I have no idea how to fix. I recently discovered that my permalink structure has changed. That means that within all my archived posts (I have nearly 2,000) all the links referring to older posts (there are MANY) are broken and turn up a 404 page.

I have no idea if I did this without realizing the impact it would have (can’t remember, but it’s possible when I was tinkering with settings) or if something else changed, but the outcome is the same: major issues with links within posts.

Is there ANY way to fix this without going back through each individual post, looking through the links and fixing them manually? It would take me forever to do that.

I even tried changing the permalink structure back to the way it used to be (month and name) but it doesn’t help – I still get the 404 error.

Can anyone help??

Thanks so much in advance.