Yes the eJ options can be converted to 320's or 321's at a later date if required.
Having said that, the 319 ordered by eJ is a 'new' variant. It will have the 320 centre section (2 overwing exits on each side) and a re-jig in the galley / toilet areas (don't know the technical details) so will come in a 156 config, with equal or better seat pitch as on the ecxisting 737 fleet. That makes the sums very different!
Not sure where German Wings have got their aircraft from, but I presume they are Lufthansa spec? Probably carrying a lot of extra gear which they don't need for lo-co operations? Are they containerised?
If the above is true, then German Wings are trying to compromise rather than go back to the drawing board.