Hi Martin,
the best advice I can give you is to try it and see. Take a simple (but not too simple) WDJ app and try porting it to SAPUI5.
Be warned there is a fundamental paradigm change known as Single-page Application or Single-page Web you will need to come to grips with. http://en.wikipedia.org/wiki/Single-page_application
You will also want to "service-enable" your backend functionality in some way - possibly using NW Gateway. This will be more than a simple wrapper over your existing backend code as you will need to consider the "stateless" nature of REST services.
Cheers
Graham Robbo