Not sure if this helps... This will work if the farm boundaries share the nodes along the way. In JOSM, you can select the farmland boundary, select the nodes at each end of the section you want to alter, then Tools > Split Boundary. Select that portion, then Edit > Duplicate and a copy of that portion appears nearby. Select the problem portion, then Edit > Delete. Drag the duplicate to where you want it to be and join the ends to the ends of the original portion you left. This method may be better if disconnecting a large boundary.

Not sure if this helps... This will work if the farm boundaries share the nodes along the way. In JOSM, you can select the farmland boundary, select the nodes at each end of the section you want to alter, then Tools > Split Boundary. Select that portion, then Edit > Duplicate and a copy of that portion appears nearby. Select the problem portion, then Edit > Delete. Drag the duplicate to where you want it to be and join the ends to the ends of the original portion you left. This method may be better if disconnecting a large boundary.

Not sure if this help... helps... In JOSM, you can select the farmland boundary, select the nodes at each end of the section you want to alter, then Tools > Split Boundary. Select that portion, then Edit > Duplicate and a copy of that portion appears nearby. Select the problem portion, then Edit > Delete. Drag the duplicate to where you want it to be and join the ends to the ends of the original portion you left. This method may be better if disconnecting a large boundary.

Not sure if this help... In JOSM, you can select the farmland boundary, select the nodes at each end of the section you want to alter, then Tools > Split Boundary. Select that portion, then Edit > Duplicate and a copy of that portion appears nearby. Select the problem portion, then Edit > Delete. Drag the duplicate to where you want it to be and join the ends to the ends of the original portion you left. This method may be better if disconnecting a large boundary.