I made a simple editor that allows changing class, stats and inventory of a unit which can be found here: https://github.com/vince11/FESOV-SaveEditor/releases
You need to use FEST to decompress the save as I have not added it in the code.
Dump save file using a save manager (e.g. JKSVM) Make a back up just in case
Use FEST to decompress the save file to get "Chapter_dec" file. https://github.com/RainThunder/FEST
Run program, Click File -> Open, select the decompressed file and edit stuff
Once finished, Click File -> Save and compress the file using FEAST
Item changing will work best if each unit has an item. You can only edit a unit's item if they are holding something.
Sometimes if a unit is not holding an item it accesses the item of the unit below them (not going to break your save file)
You need the DLC loaded to access dlc items/classes
Max stats of the unit does not scale with the class as I don't have data for it, so max stats is the max stats https://serenesforest.net/fire-emblem-echoes-shadows-valentia/characters/maximum-stats/ (Cipher characters' max stats are just placeholders)
Thats All!, I am not very good at coding so feedback would be appreciated.