Hi @Darshana2
You cannot access previous value in notification body
What you can do instead is trigger event via business rule. In there you can send previous value and current value to the event using event parameters
example:
gs.eventQueue('event.name', GlideRecord, parm1, parm2); // standard form
gs.eventQueue('sample.event',current,previous.field1_name.toString(),current.field2_name.toString());
You can then in the notification access the parm1 and parm2 using ${event.parm1} and ${event.parm2} in notification body
