The short answer is that Yes, the rules will remain the same as when you booked, regardless of what changes the airlines make after that date.
The longer answer is a little more complex.
First lets get a few term definitions out of the way. The "fare rules" for a ticket are a combination of two things - the rules that form of a part of the actual fare that you bought (which normally spell out things like change fees), and the "Conditions Of Carriage" (CoC) that is the general terms that apply to all fares booked on a particular carrier (Think of it as the "fine print" of your ticket and you'll be pretty close to the mark). If you Google for the name of your airline and the term "CoC" you should find the relevant details.
"Booking" a ticket is when you go in and, well, book the ticket! However at the time of booking your ticket hasn't actually been confirmed. At that stage, even if you've handed over your hard-earned cash to pay for it, it's not actually confirmed until it's "ticketed".
"Ticketing" is what happens when the airline actually writes you a ticket. In years gone by this was the point that they actually handed you a physical ticket. Now days it's the point where they give you a "ticket number". At this point your reservation is said to be "ticketed". Ticketing normally happened within minutes of you booking/paying for the flight, but it can take up to a few days or even weeks, depending on the situation (especially if there are multiple airlines involved).
Once your reservation is "ticketed" the actual fare is locked in, so even if the fare goes up the airline can't come to you and ask for more money for the flight itself. Whether they can change any fees associated with the fare - including things like baggage fees and change fees - depends what is written in the fare rules and in the CoC. There is an important distinction here between "booked" and "ticketed" - at least in theory the rules can change between these two stages, and it's the rules that are in place when the ticket is "ticketed" that matters. In practice airlines will generally honor the rules/fares/etc at the time of booking, but there's no guarantee that they will.
Generally any change fees will be specifically written into the fare rules, including a specific amount for the fee. eg, in your example the fare rules themselves will state that the cancellation fee is $250. As these rules are locked in at the time your reservation was ticketed, the airline can not change them after the fact for your booking.
Some airlines, especially some low-cost carriers will NOT have the specific change fees within the fare rules, but will instead make reference to standard change fees. In this case it's a little less clear what they can change as it will depend on the exact wording (eg, if it says something like "at the time of the change" then it's obviously different to "at the time the booking was made").
At the end of the day a lot of this comes down to the airline you're flying. If you're flying with one of the major carriers then their systems should automatically handle the change fees for you, at the correct amount for when your reservation was ticketed. If you're flying a low-cost carrier, or a smaller regional airline, then you might have some more problems convincing them to charge you the lower amount - regardless of what is actually correct!