Is it possible to change custom group/custom field Entity Scope from just individuals to individuals and households (or ANY)?
An old question, but an answer might still be useful to someone ...

If you had set the Custom Group to extend 'Contact' then it would be available for Individuals, Households and Organisations. But you can't change the extends (Used for) setting via the GUI after creation.

Two options:

1) Create a new Custom Group used for Contacts.  Then:


go to the old Custom Group, View and Edit Custom Fields, 
on the relevant field click More then Move
select the new Custom Group.


2) Modify the extends parameter via the API. For example, use the API explorer:


set Entity to CustomGroup 
set Action to get 
Execute
find the right Custom Group and note it's id 
change Action to create 
change Parameter to Custom Group ID = the id you just found
add Parameter, Custom Group Extends = Contact
Execute


Be aware that the API lets you do things that the GUI does not, and it is possible to break your system in nasty ways with it.  Just because the API lets you change something doesn't mean you should!