Assuming Upstream as "Design View" and downstream "Manufacturing View"
MPMLink provides function called "New Branch" in MPSE in 10.2 or MPSB for 11.0+
Branches are splits in an object's version tree which enables users to work on parallel or alternate design paths. New branches can be created for parts, process plans, operations and sequences and is a key tool to use during BOM transformations, or when creating associative BOM links.
During the creation of new branches you can define various attributes for the new branch, such as:
• The view to be used for this branch (for example, Design, Manufacturing).
• The Windchill PDMLink folder location where the new object can be found.
• The type of BOM that is to be applied to this branch (for example, Production, Testing).
• The alternate BOM number that is to be applied to the branch.
• The propagation options that are used for creating the new branch, such as whether or not to use the upstream object attributes.
There is nothing in terminology of Windchill called Downstream Part, instead in Windchill MPMLink there is something called "Downstream Equivalent Part"
Equivalent Parts Tab
Use the Equivalent Parts tab to display and modify the equivalent part information of the resource selected in the tree structure display. The equivalent part tab links a manufacturing resource, with an equivalent part designed elsewhere.
This link is typically between a part iteration, and a resource iteration. The following tables appear on this tab.