FYI...i did not have success importing my mail from yahoo using the Gmail importer. It stalled after 2 days and did not progress after 5 days...But this is a workaround that will work..
So...after researching mail import between accounts and reading lots of posts and checking with my IT group, i found a simple way to transfer emails from Yahoo to Gmail. requires a little savvy..but not much:
1. download thunderbird email client http://www.mozilla.org/en-US/thunderbird/
2. Install thunderbird and setup an IMAP yahoo account...fairly self-explanatory..might need to make sure that the POP/forwarding option for yahoo is checked...in my case it did not matter...its not a pop acct anyway.
3. let the account populate...all the folders will be there.
4. you will need to open each folder to make sure all the mail has been downloaded and is there...may take several hours...I had an inbox with 3000+ and it took a good 30min for just that folder.
5. Now, setup another account...your gmail account..BUT first go to gmail and.make sure its IMAP option is turned on in your gmail settings
6. OK, let your gmail acount populate the folders (yeah I know they are labels).
7. then ... and this is very cool...just drag the folders you want from the yahoo account to the gmail account. Or you can just mark individual messages or groups of messages and drag them into the Gmail account.
8. I had some odd named folders in Yahoo and the would not drag as primary folders, so i had to place them as subfolders in the Gmail structure...but easy to reorganize and fix once you get back to Gmail.
9. Make sure you wait for all the messages and folders to copy into Gmail and load up to the server....may take a while.
10. once the dust settles and it seems like no more copying is happening (check thunderbird's status info in the lower left). you can close thunderbird and head back to Gmail...and it's all good.
11. I was a little worried i might lose my Yahoo messages, so after they donloaded to Thunderbird i closed thunderbird and I backed up my thunderbird profile by making a copy and just sticking it another folder. (the profile is in c>users>'username'>appdata>roaming>thunderbird)...but it wasn't necessary as it worked fine.
email me if you need help...