This is T-72 argument all over again though. Of course everyone knows about S-300 and Original BUK. Its almost 50 year old tech.
The whole question is about upgrading it. Russia has upgraded BUK and is working on upgrading S-300 if not yet already done so. The technological advancements in systems like S-400 or future S-500 get passed down generations below.
As I'm aware, all Russian S-300 and maybe BUKS are all upgraded tech. Based on Ukrainian performance in the tank upgrade sector - which was their bread and butter during soviet times. I am hesitant to believe that without any prior infrastructure or experience in SAM tech, Ukrainian's have managed to upgrade their systems to same level as Russians. If it was that easy, Ukrainian's would not be asking for Patriots which they are constantly turned down for.
Especially considering that majority of SAM performance is made out of radar and actual missile tech. I just do not believe that Ukrainians have upgraded their Buks to BUKM3 level that is capable of intercepting ballistic missiles.
Again as before Ukraine aren't acting alone on this - Ukrspecexport for instance works closely with other countries to upgrade military systems and refurbish older ones that are in use by other countries militaries, etc.