https://www.pelicanparts.com/techarticles/Boxster_Tech/11-ENGINE-911_Engine_Swap/11-ENGINE-911_Engine_Swap.htm
it's all in the link above. the wiring in your 2002 is already set to take a 7.8 dme (as are the abs pump, gauge cluster, wheel speed sensors, hvac controller which also updated in 2002). you'll have to add/extend wires for the updated variocam (which is not variocam plus, fyi). given all that, no reason why you couldn't use the 2003 harness to avoid updating the cam controller wiring (i don't think so, but some harness connectors may have changed so you may have to do some splicing - you won't know until you get at the engine). also realise you shouldn't trust the internet - for something complicated like this you should do your homework, get wiring diagrams for the components in question, compare terminal assignments and wire colour codes to make sure. again, most of the required wiring info is in the link above.
finally, what 7.8 dme are you getting? if for a tip, you will have to recode.