This remains the most obtuse and difficult thing I've been dealing with since switching to Quickbooks Online in 2014.  I sometimes need to change the product/service so it matches the right Income account (i.e. set it as a service for building website, but then realize, sometimes when generating tax reports, that it should be Sales of Product Income (for infrastructure resources, etc.), so I change the Product/service to get the right income account - but lose everything that has already been invoiced for the sake of accurate accounting).  Bah!  So many, many years building web systems for my customers, and yet the accounting web system I use can't implement a simple ("Do you want to clear your existing values?" prompt)...  Just, tired of this...