Break up massive INBOX?

[mrobti at insiberia.net]

I have inherited a 10+ GB mailbox which dsync converted to maildir very 
nicely. But 15GB in tens of thousands of messages is too much in one 
INBOX when used in webmail.

I guess it's best to divide the messages into subfolders for date.

Is there a server-side way to safely move messages into new subfolders 
so that Dovecot indexes and UIDs won't break? If I write a shell script 
to move maildir files based on date of the files, I think Dovecot would 
not like this?

Also, is there any way to automatically create delivery or archive 
folders by date eg., on the 1st of each month? And deliver mails to the 
subfolder for the current month? I know I can create Sieve rules for 
such by hand, but prefer if there is a plugin or something to do this 
automatically.