It depends on your workflow.
The manipulation itself would work and valset would use the new_transform when self.transform is called. However, if you are wrapping valset into a DataLoader using multiple workers, you have to be careful when (and if) this change will be visible.
When you start iterating the DataLoader, each worker will create a copy of the Dataset until the loop finishes. Changing the valset via loader.dataset.transform = new_transform would then be visible in the next epoch (or when you restart the DataLoader loop). Also, if you are using persistent_workers=True, the workers would never restart (and thus also never create a new copy of the dataset) and thus the change won’t be used.