even when people deliberately try to feign some aspects (e.g. switching writing styles for different pseudonyms), they will almost always slip up and revert to their most comfortable style over time. which is great, because if they aren't also regularly changing pseudonyms (which are also subject to limited stylometry, so pseudonym creation should be somewhat randomized in name, location, etc.), you only need to catch them slipping once to get the whole history of that pseudonym (and potentially others, once that one is confirmed).
But on a serious note, what did "la" mean in your context? I've never seen this.
but, those changes are usually pretty gradual and relatively small. thats why when attempting to identify someone via writing, you look at several aspects of the writing and not just word choice (grammar, use of specific slang, sentence length, paragraph structure, punctuation, etc.). it is highly unlikely that all aspects of someones writing changes at the same time. simply removing "ha" is inconsequential to identification if not much else changed.
additionally, this data is typically combined with other data/patterns (posting times, username (themes, length, etc.), writing that displays certain types of expertise, and more) to increase the confidence level of correct identification.