Macro to change text in a column

Can I use a macro to change text in a cell?
As an example, I have this list of names in a column. I'd like every other name to have a semicolon instead of a comma after the name.

My list has commas now
Tommy,
Joe,
Warren.
Billy,
Bob,
George,

And I want it to look like this.
Tommy,
Joe;
Warren.
Billy;
Bob,
George;

Since I'm using macros on this page, I'd like to use a macro to do this, but any ideas would help.

Thanks

Whoops, I should have said that there are other commas in the cell. It's only the end comma that I'm trying to change.

A cell might say Tom, Joe, Sam,

And I'm actually wanting the semicolon every 300 rows