This can be modified using the Solution export and re-import (un-managed) functionality.
1. Save this dashboard as a new dashboard. (Use the Save As button on the tool-bar)
2. Create a solution containing the new dashboard.
3. Create a new solution, add the new dashboard to it, and export this solution.
4. Under the dashboards node in the customizations.xml, reach the dashboard xml (FormXml node) and change the columns attribute of section to "111" as shown below:
<sectionshowlabel="false"showbar="false"columns="111"
5. For each cell, the current "colspan" is in the following order: 2,1,2,3,2,3. Change this to 1,1,1,2,1,2. For example:
<cellcolspan="2"rowspan="24"
6. Save the file and zip the solution again to re-import back.