Can you use the "Dedechange" Ability when you play Dedenne GX from your hand while Power Plant is in play?
No, you cannot. Dedechange does not activate until Dedenne GX is on the Bench, at which time it is considered "in play" and therefore blocked by Power Plant.
Source: TPCi Rules Team (2019-08-22)

Relevant cards

Note: All applicable cards may not be listed below.
SM10_183
SM10_183
SM10_195
SM10_195
SM10_195a
SM10_195a
SM10_219
SM10_219
SM10_57
SM10_57