SINGAPORE - If you're planning to fly Singapore Airlines (SIA) or SilkAir, book fast.

From Jan 20, new rules will kick in that could lead to fare increases for some travellers.

The one change that will likely affect many economy travellers is a new charge - a minimum of US$5 (S$6.70) per flight sector - that will be imposed for those who wish to pick their seats in advance.

The new charge will apply to those who buy the cheapest, discounted seats, SIA and SilkAir said in a joint announcement on Friday (Dec 15).

However, seat selection will still be available in advance at no charge for families with children 12 years old and under in the same booking.

Those who do not want to pay the charge can pick whatever seats are available when check-in opens, typically up to 48 hours before a flight.

The tweaks will also see a reclassification of fare types to Lite, Standard and Flexi.

Customers will also have three seat types to choose from: standard, forward zone seats which refer to those near the doors, and extra legroom seats (previously known as preferred seats), which are near the emergency exits, for example.

Complimentary advance seat selection will be available regardless of fare type for PPS Club members and some KrisFlyer Elite Gold and KrisFlyer Elite Silver members.

The new fare rules also include changes to mileage accrual for some travellers.

For those flying premium economy, mileage accrual will be reduced from 110 per cent to 100 per cent for those who buy standard fare tickets and increased from 110 per cent to 125 per cent for those who pay the highest Flexi fare.

Mileage accrual will increase to 150 per cent for Business Class travellers on the Flexi fare type and set at 125 per cent for Standard and Lite fares.

For Suites and First Class, mileage accrual will increase to 200 per cent.

There will also be changes to cancellation and fare refund policies, to discourage customers from cancelling at the last minute.

This is to allow the airline to better manage its seat inventory and to provide more time to sell seats that would otherwise be empty when customers cancel at the last minute, experts say.

The changes are clearly intended to help SIA boost revenues in a challenging environment, industry analysts said, noting that overall fares are down to where they were a decade ago.

Travellers like Mr Ian Zee, though, are not happy.

The 33-year-old technology analyst said: "I'm already paying a premium by flying SQ and now they are levying an extra charge, like a budget carrier. If I want to fly budget, I can just fly a budget airline."

Despite some unhappiness, experts do not expect the changes to have a significant impact on the demand for SIA flights.

Dynasty Travel's director of marketing communications Alicia Seah said: "The travel bug is here to stay. Travelling two, even three times a year, has become the norm for many Singapore residents, including those who fly on SIA. I don't expect that this will change with the new rules."

Scroll the PDF below to see the key differences for each fare type: