This question already has an answer here:
In September 2016 I bought a Pi-top with its Pi3B, which I assembled and it runs with the Pi-top software recommended at the time. I recently bought two Pi3B+. Expecting complete computability, I simply swapped the PI3B for a Pi3B+, using the same SD card and same everything else. However, it did not boot. I got the multi-coloured screen the normally briefly occurs, and the red led, but then nothing else. I tried the other Pi3B+ with exactly the same result. Putting the original Pi3B back made everything back to normal. Is there any reason my simple swap should not work? Thanks.