Can you change multiple price set selections at one time?
Probably the simplest approach would be to


find the participants you want to change
export those participants with the following fields: Event ID   Participant ID  Participant Status Id   Fee level   Fee Amount
Open the resulting CSV file and change the Fee Level column accordingly. You may need to experiment a bit if you have multiple fields in the price set, but if your selection is a simple radio button, just use the label for the selection you want to change it to. The export file will have the amount selected after the fee label (i.e. "Full Fee - 1"), but your changes should only use the label exactly as on the price set "Full Fee".
Also change the Fee Amount accordingly.
Save the file and Import Participants using the Update option for duplicates.


Note that this will not update any related contributions or payments.