You cannot say if this is a substantial change or not, because you need to know by how much the groups actually differ on average, i.e. you need the unstandardized effect size, expressed as a mean difference in the scale sum scores, or as an actual percentage of symptoms reduced, or etc. In general, there are monstrous issues with standardized mean differences, even setting aside the interpretability issues [1-3].

See also my response to GP.

[1] https://journals.plos.org/mentalhealth/article?id=10.1371/jo...

[2] https://bpspsychub.onlinelibrary.wiley.com/doi/abs/10.1348/0...).

[3] https://www.tandfonline.com/doi/abs/10.1080/00031305.2018.15...