transponder modifications

It would appear that the only modification to the transponders is to the RF front end (antenna, transmit amp, receive amp, filters, diplexer, mixer, local oscillator) so that it operates at 900 MHz (instead of 400 MHz).

All the rest of the transponder (multiplexer, de-multiplexer, coder, decoder, digital logic), where most of the patents/intellectual property are, would not be touched.

The RF components are lower cost to supply at 900 Mhz as they have been mass-produced for mobile phones.

So, I would expect the one-off cost of modification to the transponder to be insignificant when compared to the overall savings due to lower cost components to make each 900 MHz device. Quite significant when you multiply by thousands or millions.