So does anybody know a way to convert Mp3pro in WMA without losing in quality and without having to repeat operation 1500 times?
Simply put, no way.
Various lossy compression algorithms (and even the same with different parameters) are going to throw away different info and introduce different artifacts. Your 64kbps MP3PROs are already of marginal quality (even if we accept the claim of quality/bitrate twice as good at that of 'ordinary' MP3 at face value); I suspect that decompressing them to WAVs and compressing again with yield unacceptable result.
After all, try with few tracks. I understand that WinAmp supports MP3PRO. Use its output plugin that writes WAV to disk (if you don't have a more convenient method of producing them), compress WAVs using MS tools and listen carefully, using good headphones. Compare with original. I will be very surprised if you can live with the result