I'm not familiar with a macro option for this which I'm sure there would be and someone who is familiar with macros and automation I'm sure could help with this.

A temporary work around might be that when you add a new task, have your list in ascending order by priority number, so it will be listed from 1-50. Insert a new blank row above the task which has priority as 1, so at the top of the list. Put this new task as priority 1. Renumber only 1 to 2, 2 to 3, 3 to 4, 4 to 5 and then AutoFill the rest of the numbered down to 50. So you would only need to manually change the first 3 or 4, then highlight that number sequence and AutoFill down and Excel will continue the number pattern.