Am changing an employee's email to POP from IMAP, but can't import the old .ost file to retrieve older emails. I already deleted the old IMAP account and created the new POP. The old .ost file is still there, I just can't open it or retrieve any messages. Even tried recreating the IMAP account and renaming the old .ost to the new .ost file name, but that just produced an error. Any thoughts?

OST files are profile specific and POP accounts don't use OSTs. Your best bet is to use a OST to PST converter tool and then you can have access to the mail. 

http://www.ostpst.ostconvertertopst.org/

Get best OST file recovery tool to recover OST file to PST file easily.Download OST to PST Converter Toolbox 
program. Without any damages you can remove all bugs from corrupted OST file and save OST file as PST in seconds: http://www.osttopstconverter.convertertoolbox.com/

Even if you dont remember previous MAPI setting then Orphaned OST file cannot be open in MS Outlook. In this case convert offline OST file to healthy PST format using a committed OST recovery tool. Because PST file can be easily imported into MS Outlook (POP3 & SMTP) and this is only a safe method to retrieve OST file data.
Get More Details: http://www.filerepairforum.com/forum/microsoft/microsoft-aa/outlook/456-ost-and-pst-problem-how-to-migrate-from-imap-to-pop

Hope, the above suggestions would be a good clarification to understand your concern in depth.

By the way, you may refer to this another comprehensive application (http://www.exchangeserverrecovery.com/) that proficiently converts OST files into easily accessible PST format without any interruption.

OST to PST Migration Tool, it is proficient software. It is easily and swiftly converts offine files into PST files. If you want to migrate your OST files then you can comfortably convert from OST to PST format.

Read more:- http://mail2web.com/plugins/addons/for-outlook-express/backup-emails/how-to-convert-ost-to-pst_1177.html