Unfortunately the tickle-who field is an indexed field and thus you can't use global change to change it.  

The only way to do this is to open up the tickle and manually change the tickle who field to whoever is now going to handle the reminder.

There are two options for change:
  • If you can, login as her and in the pop-up tickle screen open one tickle after another and change the tickle-who field.  
  • Alternately open the contact module ( clipboard with a check-mark icon in the upper right hand corner) and open the default grid.  In the Search by field change to tickle-who and go find the contacts you want to change and change them.