This paper introduces our efforts to create UPX, an XML-based successor to the venerable UNIPEN format for the representation of annotated datasets of online handwriting data. In the first part of the paper, shortcomings of the UNIPEN format are discussed and the goals of UPX are outlined. Prior work related to UPX in the form of the recently proposed hwDataset representation is presented. The second part of the paper summarizes the status of the UPX effort, in particular, experiments to map UNIPEN elements to hwDataset and InkML and identify potential issues with migrating existing UNIPEN data to UPX. This is work in progress, and we invite participation from the handwriting recognition research community and industry to make UPX a reality.