Well, if your RD is a "long cage" or SG variety and not the "Super short" SS variety you can get by without having to shorten the chain to match the 12-23 cassette.

IF you have the SS RD you would need to shorten the chain or keep the chain the way it is and be careful not to cross chain with little-little or big-big because this can cause big problems if the chain is too short and an inconvenience if it's too long.

It's all in the RD's chain slack take up capacity.

I have a 11-34 and a 12-26 cassette on my commuter and have the chain set to the 11-34. Lots of chain slack with the 12-26, but the RD is a SGS long mountain cage so it's capable of this.