Fear not. Thunderbird looks very much like OE. All the familiar stuff is in all the same places and the UI has the same basic look.
I also had a very comfortable relationship with OE but when I found out how full of holes it was I started looking. When I had to do a Disaster Recovery of my personal data and settings with OE, I cursed it to hell and switched to Eudora. When I wound up hating the Eudora interface, I switched to Thunderbird. Now I'm in love again. It is rock solid, dependable, easy to use and reminds me of Oulook Express, my old girlfr....er, I mean mail client.
Hey, just give it a try. You can use both at the same time. All you'll have to deal with is a pop up window asking if you'd like to make one or the other your default client. Once you try TBird, you'll be hooked.
G'head, g'head....take a little taste. You'll like it.