I think both cage bases are about the same size but substrate does flatten so you'll probably have room for a bit more. It's just it would be better to put the new on the bottom and his old substrate on top, so was thinking you wouldn't get to see the colours if using it straight away
But you could just have a new corner with some of the Colours in and keep the rest of the base the same I guess. But probably best to just resist making it nice and colourful for a month until he's settled in.