Chances are that you could get away with it by disabling the R&D mode but...

Enabling the R&D mode is associated with some sort of risk of damaging your device and is not endorsed by Nokia (see the scary disclaimer at http://maemo.org/maemowiki/HowDoiBecomeRoot ). No doubt, you can switch off the R&D mode easily but its traces would still be there giving away that you were playing with fire. If I sent back my 770 for warranty repair then I would avoid giving them the slightest chance to refuse the free repair, and in your case this means reflashing with a fresh official image. You wouldn't want to send it in anyway with your data on, so do a backup and flash to be on the safe side. Again, I may be overcautious...