Add the Gmail account to Thunderbird, using IMAP, then move or copy your old messages into the new account in Thunderbird. These messages will, given time, populate the folders on the Gmail server. It's a little tedious, but I'd strongly advise that you manually create the folders in the Gmail account, then you can copy or move the ''contents'' of the folders in the old account into their counterparts in the new gmail account. I'd also advocate using multi-select then right-click and "move to" or "copy to" from the pop-up menu rather than drag and drop. It may appear that you can drag and drop folders between accounts, but the results of doing it that way can be a little unpredictable. When you have done all this and all the required messages are in Gmail (confirm this in the Gmail website or app) you can simply stop using Thunderbird, or just remove the Gmail account from Thunderbird if you have no further use for this account.