I did a DIY rebuild on a Serta mattress a couple of years ago. Details here: https://forum.mattressunderground.com/t/serta-rebuild/?start=0
I found the pocket coils to be in good condition even though the mattress had sags and dips in several areas. The hard foam surrounding the coils had started to break down a bit though. I decided to go ahead and reuse the pocket coils and surrounding foam in the rebuild. A year later the hard foam had broken down to the extent that it was difficult for my spouse to sit on the edge of the bed without sliding off. I decided then to replace the pocket coils with a latex core.
If your mattress has a foam surround, does it feel firm all around? You may be able to partly open the top seam in your mattress enough do a visual check and still be able to use it temporarily by unraveling the thread as shown in the Ken Hightower DIY videos.Good luck with your project!