That's going to be a total rewrite.
This would be a good excuse to really evaluate the process. Are there any changes that need to be made? Does it REALLY do what it is supposed to do? Are there any bugs that you've just lived with that can be fixed?
The differences between the AEG and Siemens are big enough that even doing a line-by-line conversion is going to be difficult and will likely introduce even more new errors.