Yes it's possible to change the back panel but i would not be able to tell you the expenses but you can visit the nearby Samsung service center to know about the cost of replacement. And yes it indeed affects the water resistant capabilities
Quick suggestion - if the cost of replacement is too high you can use a back cover or some skin to hide it but don't expose your phone to water
If it's broken or damged it won't affect waterproof coating if it's moved from it's waterproof pasting them surely it will ... send any picture of your back panel
