This can unfortunately only be done in the database. Make sure you have a database editor on hand like DB browser (see my signature below).
Open the database (which is called theocbase.sqlite) with the editor. Note: if you use the 'open database location' button in Settings to find the database, make sure you close TheocBase before making any changes in the database.
In the 2nd tab (Browse Data) choose the 'lmm_schedule' table from the dropdown. To find the items you need to change, type the date ('week of' so e.g. 2021-06-07) in the filter field of the 'date' column. To speed things up enter the talk id into the 'talk_id' filter field. The talk id for student bible studies in this table is 100. This narrows it down to one record, in which you now have to replace the 'study_number' value. Repeat for all records that need changing and save when closing.
If anyone uses a S-89 remember that this does not hold source material or study#, so the student needs to be notified separately.
If you use mail or use the copy assignment function to send assignments in a messaging app, the source material will be sent. So you may include a warning in the source material itself (e.g. Note: this is change from the workbook)