changing a proposal from price per hour to total price

Hello,  I submitted a proposal to a buyer who had offered their project at a per hour rate, so my proposal included confirmation of the per hour rate.  Now it seems more appropriate that we mutually agree a total fixed cost for the project.  How can I change this on my proposal so that the correct amount will be deposited and invoiced? Do I have to delete the current proposal and send a new one?  If I do that will it break the chain of discussion that we have had?

Thanks, Emir