A contract cannot be unilaterally amended or changed after it is formed.
(So if you formed a contract asking for parts to be made, but did not specify quality or materials, the manufacturer is not bound by any subsequent specifications you retroactively impose on them).
If the manufacturer agrees to such a change (usually requires an increase in manufacturing fees), then this is an amendment of the original contract (a "new" contract is formed), and now you can hold them liable for the new specs.
You will need to draft your own opposition to the demurrer (there is not a form for this), you can find templates (examples) of these in your local law library, but this is a legal argument paper that requires some research and argument put together based on the facts of your specific case.
If you plan on filing an amended complaint to cure the defects - contact the opposing attorney first (do not just file the amended complaint), if they agree ("stipulate") to you filing the amended complaint, they will remove the demurrer from calendar, and you can file your amended complaint.
But in most cases, (and I suspect what is going to happen here), the opposing counsel is not going to withdraw the demurrer unless you can put together a complaint that deals with what they see as a very patent (obvious) defect in your claim. You will need to discuss this with the attorney (I do not know how cooperative they will be, attorneys attitude towards this type of issue vary widely).