Help with changing multiple =SUM column references

Hi - wondering if someone can help me.

I have a financial spreadsheet where I have monthly results, and every month I make a copy of the previous sheet and then fill in that month's "actual" vs budget column.

I have one column which looks at a tab containing 12 columns of LAST YEAR's results, and for each monthly tab I want to show the "YEAR TO DATE" info from last year.

So each of those cells references the other worksheet tab and is a =SUM('LAST YEAR'!E13:i13) where E13 is "January" and then the second reference is to the same column for the month I'm in....in this case I13 gets me to January->May in that reference tab.

So next month when I copy the sheet and then turn it into "June", I have been having to go down that column and hit F2 and delete "I" and type "J" into each formula so it will then be referencing 6 columns from the reference sheet.

Is there a way that I can set up the =SUM formula to pick off the column reference from a single text cell somewhere....so I can just change "I" to "J" on that sheet and then all the formulas will then pick off =SUM E:J.......

I would just change it once and then use the mouse to fill down, but some of the cells are taking just 1 row from the other sheet and others are taking 2 rows and combining, so if I copy the top formula all the way then I lose those differences.

Any ideas?