1)

I know if I modify a Part that will be reflected in the Assembly or Drawing, but what if make an alternative Part under a new file name, keeping the original file intact?

I assume keeping critical dimensions so it will still fit; but altering for example the outlines for a better way to make the physical item.

Can I replace the first with the second without the error-prone rigmarole of deleting the Part and importing the new one? E.g. by changing the file-name in Design Explorer from, say, Link vA to Link vB? I tried to do this but eventually could only delete the first and install the second, editions of the Parts.

I know if you open an Assembly it sometimes lists the Part files but I thought that’s only if you’ve moved or deleted one, or changed its name, so the Aseembly can’t find it.

…..

2)

The other query relates to opening Alibre Atom.

Have I mis-set something or have I broken it?

It opens in New Part in Model mode, the 2D sketch mode does not open because it asks for a plane but I cannot select one. The side of the screen shows this: