Media file url missing after server migration

I have recently had my wp 3.2.1 site migrated to a new server
Now almost all the thumbnails are missing

And I am NOT able to upload images to the wordpress media library

I get his error “image.jpg” has failed to upload due to an error
Unable to create directory /home/OLD_hostingaccountusername/public_html//wp-content/uploads/2011/12. Is its parent directory writable by the server?

IN FACT THIS IS THE OLD SERVER AND USERNAME that the media uploader is trying to upload to

Uploads SHOULD be going to the new server: /home/NEW_hostingaccountusername/public_html/wp-content/uploads/2011/12

My web hoster (Arvixe) has told me “check the script configuration and make changes to the account path and update the user from OLD_hostingaccountusername to NEW_hostingaccountusername.”

I will have a freelancer do this but in the meantime could someone be so kind as to point me to some documentation or solution so that I have an idea of what amount of work is involved?

Thanks in advance

Mark