Mr Nelson, I am afraid booking wouldn't be possible under any circumstances. Neral and CST, under railway parlance comes under similar set of groupings ie the suburban railway and hence a reservation is technically not possible between these two stations. You gotta either take your chances and trust the ticket checker to not have stuck to the rule book (which is common in India), in which case you get your seat at Neral OR you can cancel the present one, and take an unreserved 2AC ticket at the ticket counter at Neral. You can then hop on to the 2AC coach and sit in any empty berth available. And I am sure you will find berths in this train.
Alternatively I...
more... would suggest you to buy a first class ticket and take any fast local train to CST. Around noon on a weekday/weekend, the first class coach would be fairly empty. You have dozens of fast locals from Neral to CST which takes about 90 mins, the same as Sahayadri exp. Here is the list-
click here