When trying to switch weapon attachments while in game I come across the following problem:
While I can remove attachments that are already on my weapon and also mount them back on the same weapon and other weapons that support them, I can not mount attachments that haven't been used on weapons that support them.
The slot in the inventory remains red and dropping it in there does nothing.
Dropping "used" attachments into a crate causes them to group together with other used attachments, effectively creating two slots in the crate for the same attachment.

You can see that I have the unused holosight selected with my mouse hovering over my inventory in the screenshot. {F17286} {F17287}