Legends you allready own and crafting?
You are still able to complete the collections, and yes you would have to actually complete it, it won’t fill automatically just because you had a precursor before.
But the old precursors can be sold, even if they’re made through collections. So you could make it and then sell it instead of completing the legendary. Although there’s a bit of a debate about whether you’ll lose money doing that.
“Life’s a journey, not a destination.”
It won’t give you the collection just because you have it from before. It doesn’t give you mastery points. It give you 9 achivements points per precursor. Like Danikat said, you could for the old ones complete them and sell the precursor after. It will probably take times and you will probably lose gold for most of them. But If you want the point, I guess it won’t be that bad if you are really into achievement points.
Regardless if you already own the fully realized Legendary version of your precursor you will start from scratch on the collection and will need to complete 100% like everyone else who may not own the Legendary.
So, what do you do with the one time craftable Precursor per Legendary? That’s up to you. If it’s a Tyrian precursor you can craft it and then sell it right on the TP or upgrade it to its Legendary form to…well…sell on the TP as well. Other than that you get a few achievement points. Though I would take caution when deciding to craft a precursor as some pre’s have costs (factoring in time) that surpass just going to the TP and buying the pre itself.