It is tempting, when changing batteries in something like a flashlight, to just change a single battery, even if the device requires two or more
Not only will not changing all batteries at the same time reduce the performance of the newest battery, it may also damage the device by causing the older batteries to rupture and leak.
Here is the recommendation from Duracell, the maker of ProCell batteries.
To Duracell’s credit, this is not advice intended to sell more batteries. If you replace some of the batteries in a device, you will actually shorten the life of both the new and old batteries, increasing your costs and increasing the likelihood that you may actually have to replace the device itself if the batteries leak.
For longest life in high consumption battery use devices, use ProCell batteries, Duracell’s line of alkaline batteries for professionals.