Well, Americans did do similar things occasionally (though fortunately their system is mostly consistent).
F-5 Tiger to F-20 Tigershark for example, though in this case the designation was a marketing trick really, and the plane ended up staying mostly on drawing boards with no buyers.
And then there's the Super Hornet, though it is is more like a new plane with old designation.
Keeping track of Russian designations is difficult for sure. I assume it is marketing ultimately, higher number is usually more advanced so...
Like, the PAK-FA got Su-57 designation. Though i guess part of the reason could be that the first number is "generation".
For example Su-27 updated to more capable Su-33/35 (or whatever it was), then there's the more advanced but never produced Su-47 from 90s, and PAK-FA/Su-57 represents another entirely new generation.