Robert,

sry for my ignorance i didn't saw your earlier comment

use below script in business rule:

if (current.company.name == 'company1')

        current.company = sys_id of company 2

Using Client script:

1. var cs = new GlideRecord('sn_customerservice_case');  
2. cs.addQuery('company.name','=','Company 2');  
3. cs.query();

if (g_form.getValue('company.name') == 'company1')

        g_form.setValue('company',cs.sys_id);

for background:

1. var company2 = '';
2. var cs2 = new GlideRecord('sn_customerservice_case');  
3. cs2.addQuery('company.name','=','Company 2');  
4. cs2.query();  
5. if (cs2.next())
6. company2 = cs2.sys_id;
8. var cs = new GlideRecord('sn_customerservice_case');  
9. cs.addQuery('company.name','=','Company 1');  
10. cs.query();  
11.  
12. while(cs.next())  
13. {  
14.  
15. cs.setWorkflow(false);  
16. cs.autoSysField(false);  
17. g_form.setValue('company',company2);
18. cs.update();  
19. }

Follow the step properly and you should get the result.