If we need to change the basepoint for a block, most of us would Explode the thing, then Redefine the block. My friend, Ben Senior (tech editor, extraordinaire), suggested this very straightforward solution.
How to Use Block Editor to Change a Block’s Insertion Point
1. Single click on the block with the misplaced basepoint, then click Block Editor from the shortcut menu. If your right-click isn’t set up that way, type BE, then select the block and press [Enter] to open the Block Editor.
MEMO: If it’s not an attributed block, you can double-click on the block to open the Edit Block Definition dialog box, then press [Enter] to open the Block Editor.
2. By default, in the Block Editor, the UCS icon is placed at the Base Point of a block. With that understanding, enter UCSICON, then OR for the ORigin option. This will position the UCS icon where the Base Point is located on this block.
MEMO: Unfortunately, in the Block Editor, you cannot manipulate the UCS icon using grips.
3. Launch the Move command, select the objects, and specify a base point where you want the Insertion point of the block to be.
4. For the ‘Second point of displacement’, enter 0,0.
5. Click Close Block Editor and Save the changes. Click on the block to see a grip at the new Insertion Point!