Evolution of a specification

The specification has evolved and is going to cost additional developer time, however, this is essential as two competitive releases have arrived in the app store recently, having checked them out they both get ok reports although neither do what ours will do... we have tweaked our specification so that one key element that one of the competitors can do functionally, ours wil now do.  I checked it out and it was a real pain to use the competitors app on the phone, something which i hope the painstakingly long time I spent creating visuals and re-creating them, to make it as easy and intuitive an interface as could be, will show this.  The spec evolves, which makes the web app evolve, to cope with the eveolved iphone spec, the cost evolves too!