Switching to a theme with 1 widget sidebar from a theme with mulitple sidebars disables widgets in other sidebars

[apeatling]

This was in WordPress MU release 2.6.2 - but it's the same in WordPress 2.6.2.

To reproduce:

1) Use a theme with multiple widget enabled sidebars.
2) Add widgets to all sidebars.
3) Switch to a theme with only 1 sidebar.

Widgets in sidebars other than the first sidebar are still registered as "in use" so you cannot access them or add them into the first sidebar instead. You cannot get to those widgets to remove them from the other sidebars as those sidebars no longer exist in the currently activated theme.

A few solutions:

1) Put a "Remove" link on widgets in the left hand column when they are in use.
2) Remove widgets automatically from sidebars that no longer exist in a new theme.
3) Reset widgets when you change themes.

This is all in "Design > Widgets" obviously.

Couple of forum threads with this issue: ​http://wordpress.org/tags/trac7851

See #9511