I've taken some notes as I've been comparing each newly-changed file in 0.42 with the 0.40 version and adapting my mod files.
What I've found to be necessary for 0.42.x for major mods:
-Look into the plant files. Your plants will need to be modified for the paper reactions.
Add code fragments:
[MATERIAL_REACTION_PRODUCT:PRESS_PAPER_MAT:LOCAL_PLANT_MAT:THREAD] >> into the structural material template of plants that have thread
[STATE_NAME_ADJ:SOLID:x]
[STATE_NAME_ADJ:SOLID_PASTE:x slurry]
[STATE_NAME_ADJ:SOLID_PRESSED:x paper]
[PREFIX:NONE]
[REACTION_CLASS:PAPER_SLURRY] >> into the thread material template
[STATE_NAME_ADJ:SOLID_PRESSED:papyrus]
[PREFIX:NONE]
[REACTION_CLASS:PAPER_PLANT] >> into the structural materal template of a papyrus plant (paper made directly from plant)
-Look into the creature and material template files.
-All creatures that have a skin layer must use a parchment material.
-Add the parchment material reaction product to skin. Copy over the parchment material template to your material templates.
-Add the inebriation syndrome to alcohol.
-Add inebration Syndrome dilution tag for alcohol-loving creatures
-Add [STRANGE_MOODS] for playable entity creatures
-Some syndrome immune creature tag usage have been corrected in 0.42
-Add [LOCAL_POPS_CONTROLLABLE] and [LOCAL_POPS_PRODUCE_HEROES] for non-aquatic beast-men, or your equivalents (for the c_var), and other sentient creatures where deemed appropriate
-Look into inorganics
-Add [IS_CERAMIC] to all produced ceramics
-New inorganics are quicklime, milk of lime; copy them over
-Add [REACTION_CLASS:CALCIUM_CARBONATE] to flux stones, except for dolomite
-Helms are no longer shaped, for some reason
-Add all the new writing-related tools in item_tool
-Copy over the NAME_BUILDING_LIBRARY and NAME_FESTIVAL tokens from Language_SYM if you're using custom language files, otherwise just copy all the new files over
-Look into entities.
-The [ADVENTURE_TIER:], [INDIV_CONTROLLABLE] tags are deprecated, replace them with ALL_MAIN_POPS_CONTROLLABLE
-Replace CIV_CONTROLLABLE with SITE_CONTROLLABLE
-The [INSTRUMENT:] tag is retired. The default instruments are also retired, so get rid of the item_instrument file.
-Add new book-making tools
-Add [SELECT_SYMBOL:TEMPLE:NAME_BUILDING_TEMPLE] and [SELECT_SYMBOL:LIBRARY:NAME_BUILDING_LIBRARY]
-Add the [SCHOLAR:] tags as deemed appropriate
-Add the new jobs: [PERMITTED_JOB:PAPERMAKER] , [PERMITTED_JOB:BOOKBINDER]
-Add the cluster of new reactions (from MAKE_QUICKLIME to BIND_BOOK)
-Add the KNOWLEDGE value
-Look for GENERATE_... instrument-generating tags at the end, add them as you see fit to your entities
-Copy over all the new reactions from reaction_other (starts with make_quicklime)