Most SIMs' expiry can be extended by simply making a top-up. For normal SIMs a single 10 baht top-up at a machine (found outside every convenience store) adds 30 days expiry, with a fee of 2 baht, so 12 baht in total. The act of topping up does not necessarily extend the enclosed data plan. Keycodes (USSD) for extending expiry often have additional requirements, like being a customer for at least 90 days, which you may not meet.
How many days, beyond the original eight were you planning on extending?
The challenges become: adding the proper amount of money, and extending the data plan, or subscribing to a new one.
Some Tourist SIMs actually have 30 days expiry, but have 7, 8 15 day expiry on the mobile data portion. AIS for example...
AIS TRAVELLER SIM
You can also add one day, 10 days internet for a fee taken from your balance.
There are also gazillions of daily plans available for re-subscription.
The simplest solution might be to get a 15-day tourist SIM, or a standard SIM add value, subscribe to a single 30 day plan.
Did you have a provider in mind?
Were you planning on getting this SIM at the airport on arrival?
How many extra days?
Any budget considerations or limitations?