Can't login to my server with my SSH key

I created a new droplet and set it to use the usual SSH key on my account that I generated a long time ago with PuttyGen on Windows. I’m on a Mac now so I just copied over the PPK file thinking that I could use it to login. Turns out I can’t. I tried changing the permission of the file using chmod 600 but that didnt work. I went back to my Windows machine and used PuttyGen to convert the .ppk to an open ssh one. Same filename but with a .pem extension. I still can’t login to my server. Im geetting this:

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@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is [a long string of letters and numbers]
Please contact your system administrator.
Add correct host key in /Users/me/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /Users/me/.ssh/known_hosts:1
ECDSA host key for IPADDRESS has changed and you have requested strict checking.

Host key verification failed.

Anybody?