This question is specific about indian railways. I've a train today evening. My ticket status is RAC33 and I don't think 33 cancellation are gonna happen in the next 8 hours. However, 73 tatkal quota seats are still available. Should I cancel my ticket and get another one in tatkal quota (and pay 200 extra)? I was hoping that they'll fill those empty tatkal quota seats with RAC and waiting list passengers at the time of chat preparation but am not sure. What should I do?
The information you have provided is both valid and correct, but just to add an insider perspective so to say (though technically I am no longer employed in the Indian Railways). I will also try and add a scenario where your thought process can backfire for others.
Indian Railway has a (in)famous quota system which is complicated but in all fairness works fine for the requirements. Examples of such quota are
Headquarters Quota - Allocated by the Commercial Manager at his discretion
Wayside Station Quota - this is almost non-existent but was given to those stations which do not have computerised reservation system, a legacy from a bygone era of manual reservations and telegraphic messages.
These two are relevant here there are others which you can lookup here.
Why are they relevant, because in addition to the vacant seats that are visible to you as unbooked Tatkal tickets these are two "quotas" of tickets that are "pooled" together at the time of "Charting", so the process of charting involves releasing any un-booked wayside station quota (in some cases) and un-booked headquarters quota to the common pool, then comes tickets that aren't booked in Taktal quota. There could also be seats from other quotas being released to this (say a Ladies quota ticket, though such cases are less lilely).
There could be waitlist/RAC in the general pool whereas there are seats vacant in another, all of these are pooled together. Anyone with a confirmed accommodation is obviously given their share, the others fall in one long queue and as per their chronology of booking, they are allocated seats, so the chances of you getting a confirmation were actually higher than what you thought originally.
Having said that, in a train formation, when there are some coaches that require maintenance, the staff wait for opportunities to remove one or more coaches from the train to maintain them, in such scenarios where there are more than 72 berths free (in case of Sleeper class) technically one full coach would run without any passengers while the other coach requires maintenance, as this would not make full sense, they can remove the coaches and you could be "stuck" in an RAC ticket.
Short Answer Yes and quota's will be used to clear Tatkal Tickets.
Long Answer: I had a similar experience. Booked Ticket around 1 Month back and got WL5. Then till two days before the train it had moved only one spot. I was about to go for booking tatkal the next day. But then based on reading on the net and other i decided not too.
When tatkal window opened it had 12 Tickets in that Class (2nd AC) our Waiting list now was RLWL4. When i checked normal ticket status (i.e. trying to book a new ticket) it was WL6. Meaning total 6 People in waiting and mine ticket at 4th. Since there were 12 Tickets in tatkal and most of them were non booked i decided not to book tatkal.
Now 6 Hours before the train Tatkal tickets remaining were 9. As soon as chart got prepared Ticket was cleared.