All military equipment getd upgraded over time. Modern military fighters undergo block upgrades during production all the time. Just look at all the F16 blocks, Typhoon Trenches and Rafale initerations.
The issue of logistics and commonality of parts is actually a non-issue in this case.
Indeed, it would be nonsensical to continue buying more obsolete equipment (which will ultimately need to be replaced anyways) when you have a better version available for the sake of parts commonality. That would be wasting hundreds of dollars to save a few pence.
For smaller, incremental upgrades, it is usually the case of a newer, better version of the same model you are already using. In which case the existing fleet could be brought up to the new standard at modest cost, and will enjoy significant parts commonality with the older models.
Big upgrades to an entirely new system usually only happen when the new system is significantly better that what you are already using. In which case it's better to upgrade early, when you only have a few dozen of the older version installed, as opposed to hundreds.