maybe i'm missing something but if you open the table up, select the column
& use edit / replace for a comma replace with a space in any part of the
field, won't that give you what you want?
IF you ALWAYS have a comma at the end of last name and don't have something like
Spencer, Jr, John
AND if you have the fields already constructed
THEN you can use an update query that looks something like the UNTESTED one that follows.
UPDATE [YourTable]
SET [LastName] = Left([FullName],Instr(1,[FullName],",")-1),
[FirstName] = Trim(Mid([FullName],Instr(1,[FullName],",")+1))
WHERE [FullName] Like "*,*"