Why is it important? "gaps" often appear in records with auto-entered serial numbers for a number of reasons but these rarely cause an issue for how your database functions so long as the values remain unique.
But I'm not sure that I follow what you are trying to do. If you don't want to renumber records, won't you have to add this new record with the next number in the series? Wouldn't that happen automatically if you just added a new record?
PS and if this "sequential number" is a field used as the table's primary key--the unique identifier field used to link it to other records in relationships, then you most definitely DO NOT want to change the value of this field for any existing record.
The records are diagrams from a technical report and the sequence of the records follows the narrative of the report.
But if you skipped a record aren't all of them numbered incorrectly? Won't you have to renumber all of them? That is easily done by the way, using Replace Field Contents and its serial number option, but you indicated that you didn't want to renumber all of them and this is the part that has me confused.
Thanks for your suggestion re replacing field contents. I'll try that out.