Infinitely growing Mail/Incoming???

[dave-mlist@bfnet.com]

I have this snippet in my .gnus file:

; Put the "Incoming*" files here
(setq mail-source-directory "~/Mail/Incoming/")
(setq mail-sources '((maildir :path "/home/dave/Mail/test/Maildir/new")
		     (maildir :path "/home/dave/Mail/misc/Maildir/new")
		     (maildir :path "/home/dave/Mail/mlist/Maildir/new")
		     (maildir :path "/home/dave/Mail/personal/Maildir/new")
		     (maildir :path "/home/dave/Mail/default/Maildir/new")
		     ))

I also have this line:

(setq mail-source-delete-incoming nil)

Unfortunately I see my ~/Mail/Incoming directory growing without
bound.  I tried to clean it once, but it caused Gnus to have a fit and
toss its cookies.

I have just changed that line to 

(setq mail-source-delete-incoming t)

in my .gnus file, but what will happen to the old files?  Can I clean
them safely?  What is the right way to do that?

Thanks,
Dave