For example if we need to redirect
x.domain1.com to x.domain2.com
y.domain1.com to y.domain2.com
and domain1.com to domain2.com
Use
#RewriteCond %{HTTP_HOST} ^(.*)domain1.com$ [NC]
#RewriteRule ^(.*)$ http://%1domain2.com/$1 [R=301,L]
but if we dont want to redirect domain1.com to domain2.com and only the subdomains then use
Use the following code
RewriteCond %{HTTP_HOST} ^(.*)\.sio-india\.org$ [NC]
RewriteRule ^(.*)$ http://%1.sio-india.com/$1 [R=301,L]