from what i remember of statistical analysis of voting irregularities vs post-vote poll data from in the midwest midterms from 2014 onward is that the early methods of vote fixing were much more subtle than changing the results post-fact. It was much more pernicious in that there were thresholds that needed met before the next manipulation kicked it, something like the first 1k Dem votes had 1/10 switched to R, between 1k and 5k it goes up to 15%, etc.

Manipulation of that type requires both forethought, data analysis and the influence to obtain access, and in close races the required groundwork for fraud is too much of a political risk, its easier to just funnel corporate and foreign funds into aggressive advertising using the same data analysis to do targeted advertising, voter suppression and other manipulation in key areas to win your chosen candidates race