"for new I assume I can then go to Options and tell it to have serial number next value"
Do it JUST NOW, then:
1) show all records
2) select your ID field putting the cursor inside it in Browse Mode
3) menu: Records >> Replace Field Contents >> Replace with serial numbers ( initial value 1, increment by 1 ) and put a flag in the box: Update serial number in Entry Options
That what just what I was looking for. Thanks a lot it was so easy.