My Unicycle organization suggests that I should get a new unicycle. I would like to get into trials (but not intense drops and gaps) and mostly flatland and tricks. I will prob do alot of stairs and usual street stuff.
I was thinking of getting a Nimbus 19" trials Unicycle with KH seat, and then later buy a KH trials frame. (http://www.municycle.ca/nimbus-trials-19-isis-uni-p-251.html ) does this sound like a good plan?
Sure, its a good choice.
you also might want to get moments for it, before the new frame. They’[re alot stronger and wider
Definitely get the 137mm Moments. The saddle that it comes with is incredibly comfortable. Changing to the KH saddle would, in my opinion, be a waste of money.
Gene
I dont think you can just change the frame to KH because its different size seat post and bearing but im not 100% on that one. 
Im sure someone will no.
You would need a new post, but the bearings are the same.
getting a new frame is one of the most pointless unicycle upgrades one can do.
especially to a frame that costs 2/3 of the price of the unicycle in the first place.
a new rim would be a wiser upgrade IMHO
Nimbus Trials are good unicycles. Upgrade to consider are Cranks to moment, seat post to cromoly, pedals, and maybe rim. Upgrades that are nice but not worth it until later on Seat, Hub, Frame.