Or does it also include anyone who is trying to acquire new skills regardless of their initial skill level? (e.g. it could be someone who is really skilled in one discipline but wants to acquire new skills in a completely different discipline).
For example, on this White House report they state: "Workers who can benefit from upskilling are both those that are in lowwage jobs as well as those with low skills." https://www.whitehouse.gov/sites/default/files/docs/150423_upskill_report_final_3.pdf
"Or does it also include anyone who is trying to acquire new skills regardless of their initial skill level?"
Yes, it can also be used to refer to presently-competent employees learning additional skills based on new developments in their field of work.
Upskill: An Optimization-Centric Competence Management
"The need to quickly adapt and deliver at rapid pace will bring in dynamic changes and upskill on technology. Hence, people who form core teams need to be on top of these changes and upskill themselves on the needed trends/technology."
(transitive) to improve the aptitude for work of (a person) by additional training
