@Smokin Joe has given you the straightforward answer: the most prudent line.
But if you plan to run both wheels, ie as alternatives depending on terrain, say, then I offer other questions for you to consider.
It may be that the chain is currently a tad long and that, when you fit the 14-28 block the chain would actually manage large to large with the RD cage not higher than 135 degrees. In which case you could leave the chain on there for both wheels/blocks.
You don't say how many 1000km the block(s) or chain have done. If the chain has done more than 2000, a new chain makes complete sense, and probably with an extra link (1"), but establish the correct chain length from first principles, don't rely on it being correct now for the 14-24.
Finally, if in doubt go a link long as the chain at the 'longer' length will still work fine with the 14-24 as your RD will likely have the capacity to wrap the extra chain, and will quickly tell you if you have, unintentionally, gone small to small.