If I change my ISP will all the emails in my various folders continue to be available to me unchanged or will they be deleted from my PC?
It depends if the ISP is also your email provider. If it is then you will lose the messages as soon as you close the account.
The safest thing to do is move the messages from your IMAP folders to Local Folders before closing the account with your ISP. If you can’t see the Local Folders, you can enable them in Menu > Tools > Settings > General > Show Local Folders.
Where do I find the IMAP folder in Windows 10? Once copied to a local folder can this new folder be imported to emClient without problems?
When you have eM Client open, in the left most column are the IMAP and Local Folders. You want to select the messages in those folders. This cannot be done outside of eM Client.
So content in local folders will not be deleted if I drop ISP?
Correct, Local Folder content will not be deleted if you change Internet or email provider.