Thanks for the reply once again - interestingly I just recieved the following message from dreamhost support:
I am sorry for the trouble you are having with this. Looking at this
error this is what I am seeing.
But this file does not exist in this location.It is actually located here.
You will need to adjust your site software paths to point to the correct location for the CodeIgniter.php file.
So it seems that your suggestion that server/software libraries have changed could be correct. We are using open-source CMS's for all current/future web projects which seems like a good plan. This website is pretty old but alas I do not think the customer will have the funds to pay for any further development's.