Locking Cells in Excel

I have a spredsheet with 12 month column headings Jan-Dec, and a global commission rate comm_rate. The design is to illustrate the question I am asking:
There are 6 rows;
> sales_to_tm (Fn of last month)
> sales_tm (entered)
> sales_ytd ( sales_to_tm + sales_tm)
> commission_ytd (sales_ytd * comm_rate)
> commission to date (Fn of last month)
> commission tm (commission_ytd - commission_to_tm)

I want to change the global comm_rate figure from 10% to 12% in month 4, but I want to be able to freeze the cells in the first 3 months, so that the calculation will adjust itself in month4, i.e. month 4 commission will higher than usual. To do this, I need to stop the calcs in months 1..3 re-triggering when I change the %.

Help much appreciated.

Found answer at secont time of looking in help.

Copy the formula, and with paste from the toolbar, take the option to paste values.

Easy squeezy.......

If it's a one off change for month4 could you not just change the formula in the relevant cell?

post crossed with solution.