I'd say that if the drawings were part of a deliverables package and the client owns them, then the client has the right to modify as they wish. An unstamped drawing is just that, an unstamped drawing. Realistically, anyone could take the engineering company's title block and slap their own drawing in it, hence why an unstamped drawing is usually not considered a legal document binding the engineering firm or the engineer to near the level of liability a stamped drawing does. However, ethically, the only drawing that may carry the original engineering firm's title and revision block is that original drawing. The onus is on the customer to make sure that any new drawing has a new title block with their company's information and log and containing new revision information with the first entry indicating that the original drawing is the work of the original engineering company, but revisions are the sole work of the client.
If the original drawing bears a signed seal, that drawing must remain as-is, and modifications can get you into big trouble.
In either case, it's vital to maintain a master change log to keep track of changes. I guarantee you that the original engineering firm will have records of what they produced and will not hesitate to produce the documentation of what they produced to protect themselves in the event that the drawings end up in court.
So in short, I'd say:
1) If the documents are the property of the client, change away
2) If changed, they must clearly indicate where the original firm's work ended and the revisions of the client began so anyone using the print knows which entity created what.
3) Revised drawings must carry the client's title block, not that of the original engineering firm.
4) Master change logs must be maintained by everyone
5) Never change a stamped drawing. Period. No exceptions. The professional engineer will protect the limits of his liability, and you will find yourself in a world of hurt if the revision is found out, not to mention what will happen if the unstamped revision of the stamped drawing causes financial or physical harm.
Just my thoughts.