Feasibility of transferring email from Yahoo to GMail using IMAP and PC email client

A friend of mine wants to move from Yahoo to GMail. I've had a search on the web, and most of the articles seem from some time ago and/or might not work. I wonder if anyone could comment on whether the following would work?

1. Install Thunderbird (or other client) on PC.
2. Configure Yahoo to use IMAP, and set up Thunderbird to access Yahoo IMAP
3. Download all yahoo mail to PC via imap
4. Configure GMail to use IMAP, and set up thunderbird to access GMail IMAP
5. Drag and drop folders on Thunderbird from GMail to Yahoo, to copy the mail
6. Thunderbird will automatically upload the mails to GMail

Steps 4 to 6 are how I copied all my mail from my ISP-provided-email to my GMail account, so I know that this can work. I'm wondering more about steps 1 to 3, and whether there is a gotcha here that I don't know about, especially as I've never used Yahoo.

Or is there a better way?

Ensure the PC client is set to download all IMAP mail including the whole message. Most PC mail clients only download IMAP headers until you actually view the messages.

In Thunderbird the IMAP sync options look something like this:

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