The best way is to manually convert the application yourself. I have heard that Siemens has a partial converter and that they will do the conversion for you for no charge. I cannot verify that, but I have read plenty on the Siemens blogs about it. The issue is that in order to take advantage of the structure of S7 programming, you will need to do more than a "straight" conversion. The same is true about going the other way as well. The best option may be to hire a low-cost system integrator to do the conversion for you. I am going to guess that you may have 100-200 rungs in your program. Strictly as a benchmark, converted a program from one platform to another manually not too long ago. The program was 250 rungs and it took 3 days (~20 hours) to convert the program, and another day to simulate and verify the changes. It may take longer if you have complex functions like sequencers or PID loops, but even then, there are direct substitutions in the S7 world for the A-B counterparts. Good luck!