That is indeed exactly the case, thank you for pointing me in the right direction. It looks like there are only a handful of overrides with a few minor additions to the code:

• BigTreePaymentGateway

• BigTreeCMS

• BigTreeModule

- The changes to the payment gateway are no longer necessary as we are only keeping this site around for archival purposes and the payment gateway can be disabled.
- The only change to BigTreeCMS is the urlify() function now removes periods:

$title = strtolower(preg_replace('/\s/', '-',preg_replace('/[^a-zA-Z0-9\s\-\_]+/', '',trim($title))));

is now

$title = strtolower(preg_replace('/\s/', '-',preg_replace('/[^a-zA-Z0-9\s\-\_\.]+/', '',trim($title))));

I'm not sure why that was added but I think it's safe to drop it.

- The BigTreeModule override might be a bit trickier - it looks like the previous dev added a relationship function for a custom module:

getManyToMany($parent, $relation, $sort ='joinTable')

Should I just be able to copy modules.php from v4.2 to the custom/inc/bigtree folder and drop that function in?

Is there a reason the BigTreeCMSBase class can't be found in bootstrap.php? Thats the error I'm getting when I try to follow the steps listed in your upgrade manual.

Fatal error: Class 'BigTreeCMSBase' not found in line 65.