So the 404 error is a page not found error. The html referenced is what is generated to find your new "page". There are a series of files involved with adding a product type. From the sounds of it, the gold plugin addressed that for the gold product type, but in transitioning it to silver, something was missed.

Ideally, such a change in the product type would involve only changing every occurrence of the word gold to silver, wherever it was used. So within the files and in the file and folder names themselves. This is where I would start looking to resolve your issue.

Ohh, and capitalization is important in all locations changed. CAPS should stay capitalized, lowercase should remain lowercase.