Apple Mail Manual Migration 10.6 to 10.8

Question

     I am currently making the switch from Snow Leopard to Mountain Lion.  I have always manually migrated all of my data because I feel its better to give the hard drive a chance for some spring cleaning and get rid of the junk that accumulates over the years.  Apple mail seems to organize the mail files a little differently than in 10.6.  I see a V2 folder in it and I made a test account to compare with how my Mail folder looks, and they do look a bit different.
My Question:
     I read in another forum that if you simply paste everything from the old mail folder to the new one (along with relevant preference and keychain files) that mail in 10.8 will do what it needs to do to organize them.  Is this true?  If I simply drag and drop everything from my old mail folder, will mountain lion mail recognize those files and work without a problem? 

Answer 1

I'm in the process of upgrading from an early 2008 iMac with Snow Leopard 10.6.8 to a Mid 2011 iMac with Mountain Lion 10.8.3 and had a similar query but things seem to have gone smothly for me with a manual mail transfer / migration.
I have 3 email accounts but quite a lot of messages along with folders, rules, etc to filter them all.  At the very least, I just wanted to get all the messages across in tact and preferably in the correct folders.  I was willing to set up the rest manually.
I have actually managed to transfer all messages, folders (with structure), rules and mail accounts with all server details.  All I had to do was provide the 3 passwords.  The ML installation was clean from scratch (bought it 2nd hand) and I did not open Mail at all.  I don't know for sure whether this is very important but I worked on the basis that it might be.
I managed this by first rebuilding every mailbox and folder on the SL iMac.  I then quit Mail and in Finder, I navigated to the Library where I copied the 'Mail' and 'Mail Downloads' folders to a USB stick.  I also went to the preferences folder (still in Library) and copied the com.apple.mail.plist file but also came across another apple mail plist file with a lot of numbers and letters so I copied that to the USB stick too.  So I have two folders and two plist files.
On the ML iMac, I navigate to Library (with a Finder windo open, click on the Go menu, hold left option key and it will appear in the menu).  For safety's sake, I made a copy of the existing Mail folder which contained a V2 folder and some stuff under that.  I then deleted it from the Library and copied the two folders on my USB stick.  There were no mail plist files in the preferences folder (presumably because I hadn't run Mail at all yet), so I dropped in the two plist files from my SL machine.
That was it, so I ran Mail which came up with a first time use screen, then there was a splash screen informing me that it was updating / upgrading my migrated mailboxes, etc and that this may take a few minutes.  I didn't time it but it didn't take very long and then it asked for passwords for each of my email accounts.  After that, Mail opened a proper displaing all mailboxes, smart mailboxes, folders, accounts, etc.  Rules appear to be in tact.  Looking at the library now and the folder structure has completely changed with a V2 folder and so on.
This is exactly what I was hoping to achieve and I didn't think it would work but for those moving from SL to ML, it can be done.  If it hadn't worked, my next move was to upgrade the SL iMac to ML and then try to migrate that way. 
Hope it helps someone.