matsk,
I was really trying to avoid using third party software, but inevitably, I had to. This is the solution I ended up using. I'm posting it here in case anybody else needs tips when they're trying to solve the same problems I had, even if this isn't the smartest way to go about it!
1. Use OExpress to connect to the IMAP server and create mail directories to mirror the local folders.
2. Close OExpress, and install Eudora.
3. Setup Eudora to connect to the same mailbox on the IMAP server.
4. Import all of the OExpress messages into Eudora and have them converted to Eudora's format.
5. Transfer the messages into their corresponding folders on the IMAP server.
6. Close Eudora, restart OExpress. Ensure all transferred messages are there.
7. Remove all the messages from OExpress' local folders and set the IMAP server to be the default.
8. Uninstall Eudora and begin using OExpress normally.
This worked quite well, despite being time consuming. The only catch was that 95% of the messages transferred to the IMAP server via Eudora are now viewed as HTML source instead of their standard way in OExpress. But, when you log into the IMAP server again with Eudora, the same messages are displayed correctly. Therefore, I can only conclude that this is a OExpress-specific issue, and a real minor issue, AFAIC.
I stuck with OExpress as the default client simply because everyone has their own address book and filter rules already set up in OExpress, plus I found it slightly easier to use than Eudora.
Maybe with any luck, this might help somebody. The biggest lesson learned here was to go IMAP right from the start; it's far easier than reconfiguring all of your clients after 15 months of email accumulation!!