You should be able to copy the graphic symbol you can see and replace it with the Y.

Here is one way that should work.

Block just the symbol, do a control-c (press control and c keys together)
open the find/replace window.
Put the cursor in the find box, control-v (press control and v keys together).
Move to the replace box, enter Y, click replace for the first replace,
if it works, click replace all.

Remember with replace all, all symbols found anywhere will be replaced, so you will understand better what is happening if you replace one at a time.