Dear Mr Deepak,
Please follow the given below procedure for adding price history for existing plan those having no price set while adding, The tax and discount section remains as 0 if you don't want need to add them.
revenue-->advance-prepaid-->service coupon type(edit plan)-->add price class-->and save the settings.
One more thing i would like to bring into your knowledge is that if you need to change the existing price of a coupon, simply add one more price class for the coupon, It will show both prices but the recently added price will get affected afterwards.
Please find the attachment for more clarification with examples and Please do write back to us in case required further clarification on same.
Vikram Jamwal | Technical Support Engineer | Inventum Technologies
Tel : +91.120.4647000###### When replying, please ensure subject line contains the ticket number [TCKT !123456] and the recipient in the "To:" field is set to "firstname.lastname@example.org" ######