You ever worked for a large company, and you're like "oh hey this software is not as good as what our peers and competitors are using, can we switch"? Because that's followed by a bunch of meetings (assuming somebody above you indulges you and is willing to look at the software) and analysis by a bunch of. managers, IT, finance, etc.. If it makes a lot of sense and can be shown to pay itself off (by way of savings/increased productivity), has a short training cycle, and make managers look good, and it's not too much of a migration/transition, then it could probably happen fairly quickly (a year, two years?).
Now, however long that process is in the worst-case scenario, it's sooooooo much worse in the US military/government, because it has to go up the chain-of-command to the Pentagon, and contracts/paperwork/etc. have to be issued, and companies have to study the specs and ask questions, put in bids, brown-nose the military brass (and probably civilian leadership including Congress), etc. And if the software costs a lot, it has to probably go through the Congressional budget process, which means somebody has to justify it, which means that Powerpoint Rangers get involved, and people who will never use the software will have to be taught all of the specs so they can justify it. And because it's military, it's going to have to be tested with systems that have been around for many decades (we have plenty of hardware that goes back 40 years), and it's also going to have to interact with existing systems (the kind of software we are talking about doesn't operate in a vacuum) and existing hardware.
Oh, and because it's the military, any and all new software systems are probably going to be used with 5 year-old (if they are lucky) computers that have very specific specs and that aren't upgraded all that often, and that are probably limited in terms of memory, CPU, storage, etc., because a lot of those computers are either built into systems within the various vehicles involved, or they are installed on very unique hardware (not just ruggedized to mil spec standards, but things like passive cooling, etc.) without the fancy new input systems (and it may not even use a mouse or touchscreen). The military doesn't just buy hardware off the shelf for its combat arms units, because that stuff usually has to be ruggedized in some way, and it has to be extremely well-tested for a large variety of systems.
I'm over-simplifying, but I'd hazard a guess that the Ukrainians, because of the size of their military (they wouldn't have nearly the number of different integrated systems we have) and because of their unique situation, were probably able to more easily integrate off-the-shelf systems and to find some small software shop to whip up what they need. And because of their size and the lack of variety (versus say the US), if they have problems, they are more easily fixed or backed out.
In other words, the Ukrainian military is more of a smallish start-up company that can easily pivot on things like software or integrating new howitzers from the West when it needs to, versus a massive (and old) company that could be crippled by the failure of one piece of software being changed.
The US has gotten better with doing off-the-shelf stuff and integrating custom solutions more quickly as needed (and I believe the US Army group that's at UT is involved with that), but it's still a massive behemoth that does not change direction easily.