My guess is that your needing to go to new cabinet because you need more stations than the 500 can support digitally. Did you consider by adding a MGI card you can quite a bit more IP phones to the same system? This might solve your problem without the mess of having two systems networked locally (No not every feature is transparent, yes you have to now maintain two separate systems, databases ect. Little things like the times on the systems not matching will show up as annoyances)

By expanding the systems with IP keysets you can expand beyond the digital keyset limitation of the 500.

While the digital or analog max stations of the 500 is 360 the max of IP Stations is 240. The system can however support a combined total number of 600 extensions. (rel 2)