Thank you for your post.
When you add a record, any option to auto-enter a serial number will update by immediately assigning the current value and incrementing to the next value. If you delete the record, the value will not revert/reset to the previous value. Therefore, if you don't want a break in the number sequence, then replace the found set of records with a serial number, and then update the auto-enter options to reflect the next number in sequence.
Let me know if you need additional clarification.
I'd like to learn more. The only way I have figured out how to reset a serial number after deletion is run a user initiated script....so, when I open up a new record, I run a script that validates that the "new" transaction ID reflects the previous ID +1.