Source for the far right to far left statistic is Insa Consulere, one of the large public opinion institutes. These companies are so reliable that the live results on election days used to announce a winner on TV are actually from their analyses at polling stations throughout the day, not from government institutions. If the figure should change it's because these people have changed their mind afterwards, but as it stands right now, a lot of far right voters consider it a better alternative. Percentage wise, out of all parties, the far right loses the most to this new far left party. This was already predicted before any statistics were in because they just want to flip the bird to established parties.
https://www.berliner-zeitung.de/news/sahra-wagenknecht-parte...
The voter movement from center left to far right is from the official election results from a few weeks ago.