Could constipation from bad diet cause a blockage that makes feeces go from thin to thick?

It can happen: Each person with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of the food and poop through the gut, then the things to try are vegetables, water, prune juice, prunes, fiber plus water, Miralax or Movicol (polyethylene glycol), Senokot or other senna, milk of magnesia, etc... Doctor can discuss by video visit or in-person visit.