When you Archive an Article, the content is put into a state which removes it from your site as published content. The Article is still available from within the Control Panel and can be retrieved for editing or republishing purposes. Trashed Articles are just one step from being permanently deleted but are still available until you Remove them from the Trash Manager. You should use Archive if you consider an Article important, but not current. Trash should be used when you want to delete the content entirely from your site and from future search results.

You indeed can change the Menu Item's Type to whatever you want, even after they have been created.

If, for instance, you want to change the Blog Section of a Menu link, go to the Control Panel->Menus Menu->[menuname]->Menu Item Manager and edit the Menu Item. Select the Change Type button and choose the new style of Menu Item Type from the available list. Thereafter, alter the Details and Parameters to reconfigure the display for the new selection as you require it.