Every now and again, we need to take our contact list from one program or computer, pack it up, and bring it elsewhere. This happens whenever you get a new computer, an additional computer, or a new emailing program. Or you might even need it in the wretched case that your computer actually crashes and you need to start from scratch - hopefully you'll have backed things up to keep your list!
Every program has its own method for allowing you to import your address list and export it. It is important to familiarize yourself with your program so that you do know how to back up your list...just in case. Consider it an action similar to bringing an umbrella in case of rain, just as it never rains when you are prepared for it, so will your computer not crash when you've backed up.
Usually the process is quite direct, and it won't be confusing or challenging to export your list frequently to save it "just in case" or to actually use it to transfer your contacts over right away. It's just a matter of saving it into a file of its own and then bringing it right back into the new program.
Another good thing about transferring is that you can occasionally go through the entire list and keep things up to date. You can weed out the addresses that you know you'll never need again. You can also check into the addresses that you still want but that you haven't used in a long time (or that you don't remember anymore) by finding out who it currently belongs to.
You could try to use a free email search online to see what you can come up with, or you could upgrade to a premium service for much greater luck. This way you can check up on the accuracy of your list before saving it and adding it back into the program for your use again. This is the same as keeping your own personal snail mail address book up to date every now and again - only in a digital sense.