You could do something like this:
Col A Col B Col C Amt Curr Master $100 CND =IF(B2="USD",A2*1.2,A2) $100 USD =IF(B3="USD",A3*1.2,A3)
The formula =IF(B2="USD",A2*1.2,A2)
will check if column B is in USD, and if it is, it will multiply it by 1.2 (or whatever the exchange rate should be), otherwise it will display the other currency. This will change everything into CND (or whatever).
If you have multiple other currencies, you can nest the IF
like =IF(B2="USD",B2*1.2,IF(B2="HKD",B2*.75,B2))
to convert it all to the default currency.
If the exchange rate changes, you can set up the formula to reference a cell with the rate in it, but you would need to make sure that if the rate changes, you hard code the values otherwise prior values will default to the new exchange rate.