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]