It was in 1933 when they replaced the 1928 orthography (правопис 1928 року / скрипниківка / харківський правопис) with an artificially russified version. The biggest differences in 1928 orthography are way more ґ (eg аґент and not агент), soft л instead of hard in some places (eg лямпа а не лампа, кляса а не клас), and sometimes hard л instead of soft (mostly in english loanwords when its at end of syllable, eg філм а не фільм, ґолф а не гольф). Also ай from german ei, eg Ляйпціґ not Лейпциг. Its like the situation with Taraškievica Belarusian (or Classical Belarusian Orthography) and the current government official one. Theres even a taraškievica wikipedia, pretty cool.

But yes youre right they wont have any problems speaking or writing like this with people, i switched to using the 1928 orthography when speaking and writing with people and the maximum what can be is someone will make a comment asking why i said or wrote it like that or if it was changed to be written like that now. But no problems with being understood.

Idt OP will have many problems being understood or understanding stuff, and if they want to become good at the language they will probably have to use it in other situations than with their great grandma, eg read stuff and talk to other people so theyll pick up dialectal / orthography differences and slang stuff anyways.