I explain it to you;
A visa is like a new shirt. When you arrive at border control the IO looks at your shirt, the pattern, the weave and the fabric and decides whether this shirt is good enough to enter Thailand. If he lets you enter, your shirt changes its status from new to used. You are still wearing the same shirt, it's just used now, and you will wear this same shirt for your whole stay in Thailand. Some people have reason to change their shirt, maybe it became smelly after 45 days, or they are bored with it and want to try a different style, well lucky for those people there are forms that let them change their stiff-collared arrival shirts into for example more comfortable t-shirts.
I hope this clears it up for you