When you insert a new message with a new sequence number in an existing communication diagram, the sequence numbers are recalculated with respect to the following rule: for each number after the insertion, all numbers are incremented by 1.
In the same manner, a parent changes its children. For example, the number is incremented by 1 for numbers like 1.1, 1.2, 1.3 as follows: 1.1 + 1 = 1.2.
The syntax of sequence numbers currently used in the diagram is respected: that means that the number is incremented by 1 regardless of the syntax (1, 2, 3... or 1.1, 1.2, 1.3...).