Fady,
This is a very interesting and also important subject matter. To answer your question, even products that were designed and built pre 21 CFR, has to have some sort of design and technical documentation somewhere, and especially so if these products were patented. They won’t be as completes as the DHF, however, we now have a starting point. Once all these documents are identified, the next step would be to compare these docs and against the design control requirements and find the gaps. Once the gaps has been identified, the real work would begin in closing these gap and bringing all parts of control designs up to date. This will be done by the information and timeline derived from the existing documentation. The difficulty of the process will depends on the level of details and documentation that the company has retained for the specific product.

Thanks
Adrian