from reading Namepros FAQ I see they don't encourage this (I only use namepros because they are the best, easiest to deal with, no hassle, good comms and pay without fuss... so can't comment on anybody else).
To be honest, i just changed the keywords on one of my domains, and from an average of $1.50 a day or so it looks like it will make $4 (and maybe much more up to $10 a day).
However the new keyword was relevant to the domain, although not the
most relevant keyword (thats arguable though of course). My CTR% hasn't dropped so maybe the new keyword is just as relevant
Its easy for the parking admins to see abused keywords, as they'll have a very (abnormally) low CTR% and high PPC.
If parked programs find you abusing the keyword system... well... I just wouldn't go there.....