In 2015, the "Programme for International Student Assessment" (PISA) introduced multiple changes in its study design, the most extensive being the transition from paper- to computer-based assessment. We investigated the differences between German students' text responses to eight reading items from the paper-based study in 2012 to text responses to the same items from the computer-based study in 2015. Two response features -- information quantity and relevance proportion -- were extracted by natural language processing techniques because they are crucial indicators for the response process. Showcasing potential differential relationships, we additionally examined gender differences. Modelling effects of the round of assessment, gender, and response correctness on the response features, we analysed responses from 15-year-olds and ninth-graders in Germany. Results revealed differences in the text responses between the rounds of assessment in that students included more information overall in 2015, and the proportions of relevance varied substantially across items. As the study investigated the mode change in PISA's natural (not experimental) setting, the differences could mirror cohort trends or design changes. However, with the evidence reported, we conclude that the differences could indicate mode effects.