You'd need to change a lot of parts. For example the fan on the PB-255LN is an Echo P/N E100000070, whereas on the ES-255 shredder/vac it is P/N E100000064. Then you'd have to buy the cutter/shredder blade P/N X413000010. But real deal killer is that you'd have to replace the left and right fan housings and grill, because they are different as well. By the time you're done, you will have spent more than the cost of a new ES-255.
Another thing to consider that a shredder/vac is much louder than a simple blower. Your PB-255LN has a whisper quiet 64 dB(A) sound rating, but the corresponding ES-255 shredder/vac has a much louder 69 db(A) rating, which means that it is more than 3 times louder. This is why the part number lacks the LN(low noise) suffix.