Invoke this command in the group buffer if you'd like to try it.
This command fetches mails from all the Incoming* files as the
`file' type `mail-sources'.  After fetching mails, those Incoming*
files are deleted and new Incoming* files will be generated if
`mail-source-delete-incoming' is a number or nil.  Consider giving
a group level to this command like

  1 M-x my-gnus-group-get-mail-from-old-incoming RET

if you have the foreign groups that are very slow to connect.

Regards,
(I'll be absent till Monday.)