Use the following formula in the first row of the col in which you want result, and then copy and paste it all the way down to the 500 rows.
=RIGHT(A1, LEN(A1)-FIND(" ",A1,1)) & ", " & LEFT(A1, FIND(" ", A1, 1))
Let's say you have names in Col A, from A1 to A500 and you want results in Col B, from B1 to B500 - paste the formula in Cell B1 - when you see the desired result in cell B1, copy the formula from B1 and paste it all the way to B500.
PS: Later if you want to get rid of the formula and just keep the values, you can copy cells B1:B500, right click on cell C1, select 'Paste Special'. In the paste special window, select 'Values Only', click OK to paste only the values and remove the formula -- this way, your cell C1:C500 will contain names in -- last, first-- format and will be independent of values in cell A1:A500.
HTH