Hey there! As someone who's been in your shoes, I can tell you that retrofitting modern servo drives to a 15-year-old S7-300 system comes with some pretty significant compatibility challenges. Here are the key issues you'll need to tackle:

First and foremost, communication protocols are your biggest hurdle. Your S7-300 likely uses PROFIBUS DP, while modern servo drives typically support PROFINET or Ethernet/IP. You'll need gateway modules or communication adapters to bridge this gap, and these can be tricky to configure properly.

Programming software is another major concern. The S7-300 was programmed with STEP 7 Classic (v5.x), while modern drives are configured in TIA Portal. You'll either need to maintain both software environments or find a way to migrate your existing logic - which often fails due to hardware compatibility issues.

Hardware limitations are real too. The S7-300's processing power and memory might struggle with the faster communication cycles and more complex motion control algorithms that modern servo drives require. You may need to upgrade the CPU or add additional processing modules.

Don't forget about firmware and hardware revisions - even modules with the same part number can have compatibility issues if their firmware versions don't match up. And finally, documentation is often sparse or outdated, so be prepared for some detective work!

The good news is that Siemens does offer retrofit services and there are third-party solutions that can help bridge these gaps. Just make sure you budget extra time for troubleshooting and testing!