Is that indeed what Paul is explaining? Because that is not at all what the Scripture annalex quoted says. Paul calls himself a masterbuilder of this foundation, not merely a gopher. According to the grace of God that was given to him. It certainly sounds like something big had happened to me.
What of the New Covenant is new and in no way connected to the Old Testament and the Old Covenant?
What part of the promise of Christ coming was never made clear in the Old Testament?
What part of Christ being our only way to Salvation wasn't part of His being the Messiah as promised in the Old Testament under the Old Covenant?
Purgatory is a process not a place and while that's made more clear here and therefore a change from what the Jews already taught Paul isn't espousing any new doctrine not already understood even if it wasn't clearly understood. Clarifying and explaining anything that's at this point more clearly understood or altered in some way under the New Covenant is a new foundation the same way adding onto you house requires a new foundation for the addition to rest on.