[swechsler]

I have an 18 year old pair of HD 580s with one bad driver (and both have the spring contact issue that you old-timers will know about). Replacement HD 650 drivers are cheaper than HD 580/600 drivers. Any reason not to upgrade to 650 drivers, keeping everything else the same? I'm assuming the driver mount hasn't changed.
 
Thanks!

 

[GREQ]

Some people would argue it's not an upgrade, but a side-grade.
To my ears, the HD600 was more mellow, calm and relaxed.
The 650 a bit more excited sound. 
 
So if you do 'upgrade', expect a slight change in the sound. 
If you want to keep it as similar as possible, stay with 600 drivers.