Freeciv-3.3 datafile formats have been frozen. Datafiles made compatible with current HEAD of S3_3 branch will be compatible with final freeciv-3.3, and vice versa.
"How to update ??? from 3.2 to 3.3" -documents in the wiki are already mostly up to date.
Rulesets: https://www.freeciv.org/wiki/How_to_upd ... 3.2_to_3.3
Tilesets: https://www.freeciv.org/wiki/How_to_upd ... 3.2_to_3.3
Lua scripts: https://www.freeciv.org/wiki/How_to_upd ... 3.2_to_3.3
Audio file formats have not changed between 3.2 and 3.3.
Happy modding!