user8279: I am looking for any online tools that can recalculate the the FEN values based on the current FEN and a given step. Are there any such tools or should I write one on my own.

user7897: I don't think this is exactly what you're looking for, but it's at least a start.
The Analysis Board at chess.com allows you to paste in a FEN, make a move, then go to the Position Setup tab and copy the FEN.

8279: <p>I am looking for any online tools that can recalculate the the FEN values based on the current FEN and a given step. Are there any such tools or should I write one on my own.</p>
 7897: <p>I don't think this is exactly what you're looking for, but it's at least a start.</p>

<p>The <a href="https://www.chess.com/analysis-board-editor" rel="nofollow">Analysis Board</a> at chess.com allows you to paste in a FEN, make a move, then go to the Position Setup tab and copy the FEN.</p>
