I can answer some of these questions for you.

The architectural changes you read may be referring to how things were built between sites like Tikal and Chichen Itza, for example. During the Classic period buildings were built using a rough stone and earthen core and then plastered over to be made smooth. Decorations were often included that were entirely made out of plaster like stucco masks or figures. Chichen Itza, on the other hand, was built almost entirely out of cut stone with no plaster outer layer. This was most likely due to a lack of wood needed to burn the limestone to make the plaster powder. This isn't to say that the entire Yucatan was deforested, but that the people decided not to ravage the landscape further for wood for their buildings like what happened in the Classic period. Another area to see cut stone buildings would be the Puuc region of the eastern Yucatan. By the time of the late Postclassic sites were returning to plastering their buildings such as at the site of Mayapan. Mayapan was founded by a splinter group from Chichen Itza and sometimes people say it looks like a shittier version, especially with their Castillo and Observatory. This is partly due to the smaller size, but also because at Mayapan we are left with the rough cut stone lacking the plaster.

Literacy continued to survive into the Postclassic but it was much less public than the Classic. No longer did communities erect stela of their rulers depicted as god-kings nor did they have elaborately carved staircases or lintels. Most of the writing was restricted to ceramics and perishable materials like paper or deer hide. This change is reflected in the change of the political system. No longer were there god-kings that ruled and glorified themselves. Instead it was more of just a plain king with a council made up of other important families who had a say in how things were run. You can also see this change reflected in public architecture in which some areas are more open and accessible to the public than the secluded plazas with restricted access to the buildings.

Lastly I can answer the economic question. One of the largest changes was a shift from inland sites to coastal sites in the Yucatan. So while the inland cities may have been abandoned gradually, the people did not die out they merely moved to other cities. Coastal sites grew due to the Maya playing a larger role in Mesoamerican trade with places as far as Veracruz and Honduras/Costa Rica and even into the Caribbean. Chichen Itza became, for a time, the central hub of exchange due to their control of salt beds on the coast. While the site is somewhat inland, it was easier to trek to than the thick jungles of northern Guatemala/southern Yucatan peninsula to places like Tikal or Calakmul. You can see this exchange reflected in the architecture with talud-tablero style (a Tlaxcala-Puebla origin) rising in prominence. You also see a rise in markets in Postclassic sites, which have remained elusive in the Classic period so far. My advisor in my undergrad has been hunting for evidence of a formal market space in some Classic sites with another researcher. We know that people traded goods and it was not a redistributive system like early researchers thought, we just haven't found out where they are doing this exchange.