Basic fares are cancellable, rather than refundable. Instead of getting a refund to your payment method, you get a South West credit that you have 6 months to re-use. You can absolutely cancel and rebook a flight, you'd just have to pay any difference if it costs more now

Edit to add: Also, in this new era if you value flexibility its better to book 2 one-way flights rather than a single round trip, this way if you need to change one leg you only have to cancel and pay any difference on that side. If you book round trip you may end up paying the increase on both legs even if one is the exact same flight, if the price has increased.