The United States has suggested that Greece replace its old weapons with newer systems.
Here's What We Know
The question is about S-300 and Tor-M1 air defense systems. This was reported by the Greek newspaper Kathimerini. Most likely, the U.S. wants to take the Russian weapons from the Greeks and give them to the Ukrainian Armed Forces. The thing is that Greece has no right to transfer arms directly to Ukraine without Russia's permission. But it is possible to do it through third countries.
S-300PMU1 on KrAZ-260 chassis
Incidentally, Greece has recently tested a similar scheme. The Greeks gave the Armed Forces of Ukraine 40 Soviet BMP-1, and received from Germany Marder infantry fighting vehicles in return.
Tor-M1 SAM system