can i know how to prevent deactivate and activate again plugin after change theme
and save template from change
Manually copy all custom variables from the previous template to the new theme. Apart from that, no.
http://wiki.mybb.com/index.php/Plugins:Globals
Read that and learn what the variables are. They'll be something like {$pluginname} and you need to find where they are and paste them into the same template in your new theme. It is easier to deactivate and reactivate themes if you are unfamiliar with this.