Your assumptions are all correct.
When you setup a Gmail account in Thunderbird, it will make actual "Folders" for each of the Gmail "Labels". If an email message has more than one label, it will be downloaded into each folder.
Once you setup the second Gmail account and copy (or move) the messages from one Gmail account to the other, Gmail will figure out that the messages are "duplicates" and will apply multiple labels instead.
I believe Thunderbird can be configured to only download "Headers" instead of full-messages, which should lessen the initial local storage space required. But, once you have the second Gmail account setup, and you copy one account to the other, the headers and body of all the messages will have to be downloaded.
Then, as the messages are copied to the second Gmail account, all the email message headers and body's will be duplicated again in local storage.
So, starting from the size of the original Gmail account, you will need at least twice that amount of local storage space plus additional storage space for the messages that are duplicated from conversion of Label to Folder.
Of course it depends on how much storage you have used in your Gmail account, and how many emails have multiple labels, but I would think it is well within the the free drive space of an average computer.
After the transfer is complete, you will be able to delete both Gmail accounts from Thunderbird and recover all the local drive space (unless you want to continue to use Thunderbird with the accounts).
If one copy of the Gmail account is as much as you can handle, you could use this method although it bears some risks... I wouldn't recommend it unless it is your only option.
- Create the original Gmail account in Thunderbird
- Insure that Thunderbird is configured to download the entire message, headers and body's
- Wait for Thunderbird to completely download all the messages (CRITICAL).
- Close Thunderbird, and to be safe, disconnect from the Internet.
- Restart Thunderbird, and edit the configuration of the Gmail account using the log-in details of the second Gmail account.
- Close and re-start Thunderbird.
The emails should (eventually) be uploaded to the new Gmail account.
When I say there are "risks"... I don't think there is any way email messages could be lost (deleted) from the original Gmail account. The risk is that there is no real way (that I know of) to tell that Thunderbird is "done" downloading from the original Gmail account. You just have to wait a sufficient (unknown) amount of time. If you switch the account information in Thunderbird too soon, some of the email messages will not be transferred to the second Gmail account.
As an alternate (safer) method, you could use the "migration" features to move messages from one account to another. If Gmail won't allow migration from one Gmail account to another, you could configure migration from Original-Gmail -> Temporary-Hotmail -> New-Gmail. If this is your choice, be careful, because I believe this actually "moves" the messages from one account to the other (deleting from the old account).
If you have the local disk drive space to spare, the first method as you have proposed is by far the best method.