I think this is more of the question of hardware rather than firmware.
Slave 4.177 is an old one, around 8 years back, so for how long it will continue to work I'd say that it shall work as long as all the components/parts continue to work fine.
As for the increase in buffer size in the new firmware, this is probably to cater for the newer feature introduced in Comfort.
I'm not really sure exactly what the new feature is but this would simply mean that your old Slave I will not be able to make use of the new feature (if any).
The basic function of slave i.e. reporting of zones status should not be affected though.
Yes, the increase in message buffer size in Comfort should be to cater for features like Firmware upgrading which do not apply to Slaves anyway, so the version 1 slaves should continue to function as long as it does not develop any faults