Is it possible to use 24 bit A/D and D/A convertors in place of the original Neve 20 bit convertors?
Yes, but not in the original Neve IO racks as 24 bit versions of the PL11535 and PL11538 cards were never produced.
The latest Capricorn software does support AMS Neve 24 bit IOS racks as used with Libra and DFC consoles as proper recognised IO devices, but it is also possible to use any 24 bit madi IO box such as a Neve 8800 box as a general madi device.
The Neve 8800 box, or any madi IO device will work effectively as an undefine madi IO device (like a madi tape machine), and can be ported in the same way as a 24 bit tape machine.