2 years is not an unusual time between starting product development and seeing any sort of major customer uptake, and if you're in that time period, your priority really should be getting the customers rather than futzing with code migrations.

It's still the case that a bad call was made when the codebase was started, picking a language version that was already on its way out. It's like starting a new project now with Java 7 o .Net 2.0.

Of all the bad calls you can make when starting a business, picking a bad language or platform is a relatively minor one. Google was originally on Python 1.6 (or technically Java 1.02, if you go back to when it was Larry's dissertation project), Facebook chose PHP (!!), basically every Android app is forced to use Java 7, and a good number of iOS apps are still in Objective-C.