Chain length when swapping between cassette sizes (11-28 and 11-23) Currently I am running a semi compact 52/36 up front and a 11-28 at the back. I'm planning on getting a 11-23 cassette for criterium races, and have the 11-28 for hillier rides. 

Will there be any issues with chain length when swapping back and forth between the two? If I leave the chain length as is (optimized for a 11-28), will I run into any problems when putting the  11-23 in?
It should be fine. 

The chain will be a little bit longer than necessary when using the 11-23, which may give some droop if you're running bad gear combinations (e.g. small chainring+small cog), but nothing serious. 

Never run with a too short chain. A bit long is OK.