You're going to have to clarify what you mean on that. I'm not aware of any school of thought that thinks amendments to the constitution are somehow distinct or less valid. The original document provided processes for amending (i.e. modifying) it, and amendments were made under those rules. An "amendment" is literally a modification to a document; that's what the word means.

In fact, to the extent that there's a distinction, it's that amendments take priority over the previous version of the document. Obviously they'd have to; otherwise that would just mean you can't amend the original document.

According to the current version of the US constitution, which is the only one that matters, slavery is legal in exactly that one circumstance.