How does express replacement work exactly?

I have been having issues with the heart rate sensor on my iwatch and finally called for tech support today. When we finished going over everything, he set up the express replacement and explained how that all worked with the credit card hold until my watch was returned.

My question is what happens after my watch gets to them. I assumed the replacement was for the time that they were looking at the issue with my watch, but after reading the information about the hold on funds until my watch makes it to them, I started wondering if this is simply a 'replacement' that I will be keeping.

So, will they be trying to fix my watch and send it back to me if they can figure out what is wrong, or am I essentially just swapping watches permanently with the one they are sending as the express replacement and whatever they determine with my watch is irrelevant now?