20 Year old Sensor Dimmers - can they be upgraded to work with sACN?

ETC has retrofit guides and update packages for older Sensor racks. Retrofit isn't intended to be something an end user would do but you can have a look to see what is involved.

https://www.etcconnect.com/Support/Dimming-Switching/Sensor/CEM3/Other-Documentation.aspx

Depending on what you mean by "work with sACN" it could be as simple as upgrading the CEMs and backplane. If you currently don't have feedback and want to add it then that gets much more involved since your modules and the rack wiring need significant refurbishment and/or replacement.

Your best bet would be to contact ETC Support by phone so they can pull up their installation records and give you a more specific answer.