For people who purchased Q and N Economy tickets under old fare classes prior to Jan 21 for travel after Jan 21 may be getting penalized by the new fare class restrictions with retroactive T&C change.

For one thing, it looks like free advance seat reservation in prior system when seats became available (for example at T-96) is no longer available after the implementation of the new fare classes in their software. So you would be charged for selecting a seat at T-96 if you held a ticket purchased earlier even if you had no idea of upcoming fare class changes and restrictions. This would certainly be illegal under US laws if tickets were purchased in US, no?

Similarly, I wonder how the miles accrual would also be affected if the new fare structure is stingyier for the same flights.

The right thing would have been to to grandfather those earlier tickets under the current fare classes that provide the same benefits. But I am not sure there is anyone at SQ that cares about such things.