Assuming your sheet names are in the form Jan_06, Feb_06, Mar_06 etc
and you are just about to add a May_06 sheet, then currently your
formulae in the Six Month Report will be something like (for cell C5):

=SUM(Apr_06:Nov_05!C5)

When you add a new sheet (assuming it is the second sheet and the
others are in reverse sequence), then you could just do a Find and
Replace twice. Highlight all the cells in the Six Month Report sheet
(CTRL-A), then Edit | Replace (or CTRL-H) and:

Find What: Nov_05
Replace With: Dec_05

then click Replace All. Do CTRL-H again, and:

Find What: Apr_06
Replace With: May_06

and click Replace All.

This is a procedural rather than a formula solution, but it should
work.

If, instead, your formulae are in this format:

=Nov_05!C5 + Dec_05!C5 + Jan_06!C5 + Feb_06!C5 + Mar_06!C5 + Apr_06!C5

then you just have to replace Nov_05 with May_06, and the ordering of
the sheets doesn't matter.

Hope this helps.

Pete