Here is an example of what thousands of buyers of bikes with built in batteries have to look forward to in the years to come
In this case, it's not exactly difficult - take the battery out, check that it's the same as what the seller is offering and buy it. I think you're going to find that replacing your battery is not going to be as straight forward as you think, though, like everything, it's easy when you know how. If you take it back to the shop, I'm sure that they'll be able to sort it out for you when the time comes, but, of course, you'll have to pay for that.
The main point, though is that any battery of the right voltage will work on these bikes. You just have to figure out the way to fit it. It's the bikes with special-shaped batteries and those locked behind comms protocols that cause replacement problems, which often restrict the future use of your bike to either a non-assisted one or a nice trellis for your peas.