If I have a heavily damaged Basic Pokemon V in play, can I use Thorton to switch it with a regular one-prize Basic Pokemon from my discard pile?
Yes, and if the new Basic Pokemon has less HP than the amount of damage counters it is immediately KO'd and will relinquish only one prize to your opponent.
Source: TPCi Rules Team (2022-09-29)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH11_167
SWSH11_167