Let's say my stacks of fast/bulk/stack/etc inserters have the "rare" level of quality, then the long-armed recipe won't recognize these inserters as the correct ones to convert to the long-armed version.
Assembly Machines can handle it, but I can't craft them from the player's pocket.

Apologies for my english, I hope I made myself clear enough, hehe.

Hmm, I think that might be a base game limitation. Player can't craft using quality items, this applies to all items, not just inserters. If all you have in your inventory is quality iron and copper plates, then you can't craft any items that you normally would be able to.

I'm not sure if there's any way I can change that; you will need to bring your own assembling machine to convert the inserters on location

It could very well be a base game limitation, I finally gotten to a point where I'm playing with quality, so I have many hours to go and planets to visit.
Thanks for the quick response! I'll be sure to bring an assembly machine to wherever I go!
Thanks for the long inserters, it helps my not-the-smartest-brain to not overload, hehe.