Basically, I made a level. I then copied that level and want to use a different TSX and TADAA, new level but I don’t have to worry about the layout and so on.
But how do I change the source TSX? I can change the source image for a given TSX file but that just changes it for all levels. I want to be able to direct the tiles to load from another TSX, without having to redraw my level. How?
There’s no way to do this in the editor, but you can achieve it by editing your TMX or map JSON file. Open it in a text editor, find the reference to the TSX file, and change it to your new TSX.
Thank you! 
Right, it used to be possible to change a tileset reference in Tiled at some point, but the implementation was a bit problematic and it got disabled in Tiled 1.0 as a result of other changes.
Indeed editing the file will work, but I’ll also look into re-enabling this action. Probably could be done with a button alongside the “Remove Tileset” button.
Another way to replace a tileset that should currently work is to move away the TSX file, then Tiled will give an error on loading the map that it can’t find that tileset and you can pick the new one as replacement. 