I would think post-KS upgrades will all cost more than the current pledges - whether you go from Lotus to Shogun or Sumo to Shogun. Might be a question to ask FFP (you can contact them on KS).
Jason from FFP has answered this in the comments thread. There will be the option to upgrade your pledge in the manager after the campaign closes. The difference will be more than the current difference between pledges; though the exact amount depends on how many stretch goals are unlocked. This is to encourage people to go for a higher tier while the campaign is still running.