In regards to Ian & George’s original anchor system, (ropes through loops fixed to submerged concrete slabs) this worked very well for years and I believe could again. The main problems I encountered were; repaired/knotted rope catching in the loops and overlong tethers from the buoys which can cause the clips/cleats to get jammed in the submerged hoops. Particularly when they have been pulled down too much by yachting types to get the submerged buoys out of the way of their keels.
New complete lengths of rope & short tethers should resolve this.
As for repositioning them, you would need two or three persons in waders with long poles for leverage, with the rescue boat alongside as a working platform to put poles/tools, coffee flasks etc