Simply pick the old Leader then the associated text and a new Mleader is created based on location and layer of old Leader and text.
The old leader and Mtext are deleted. The AutoLISP routine is 'open source' so you can modify as you see fit, if necessary.
About This VersionVersion 1.1, 5/14/2019
Added support to AutoCAD® 2020 (No change in version number).