I'm a believer in the big/big method for determining chain length. If your chain is too short, accidentally shifting into the big/big can cause a bunch of damage to your bike in less time than it took me to type this. In your example, neither the big chainring nor biggest cassette cog has changed so, assuming the chain was right previously, it'll still be right.

On the other hand. Chains and cassette (or freewheel) cogs tend to wear together. An old chain will wear new cassette cogs prematurely. I don't always change cassettes when I replace the chain but I always install a fresh chain with a new cassette.