If you select all 50 rows at once, you can modify them all at once, but they will all end up with the same run date/time, which I understand is probably not what you want. In order for 50 tasks to all have different run times, they cannot be modified at the same time in BP. Your best option is probably to save the grid to .bps file, and then open the .bps file in a text editor like Notepad++ and manually update the run dates.