Early on when coolants changed to different types you could not mix types, they would gel. These days all coolants can intermix without issue, so you are good to go.

Here is one radiator I replaced back in the day, pulled the radiator tank to expose what happens when coolant gels. Destroyed many engines due to gel plugging radiator cooling tubes and causing overheating of the engine.

Early on when coolants changed to different types you could not mix types, they would gel. These days all coolants can intermix without issue as coolant manufacturers made coolants compatible with any other type of coolant, so you are safe with mixing these days.

But as Paul stated doing a cooling system flush is part of regular maintenance as coolant breaks down after 100-150K miles or so.

Here is one radiator I replaced back in the day, pulled the radiator tank to show you what happens when coolant gels. Destroyed many engines due to gel plugging radiator cooling tubes and causing overheating of the engine.