Simple way to convert imperial drawing to metric:
1. Click the main menu (the autocad logo on the uppermost left corner) then Drawing Utilities then Units.
Change the Lenght Type from Architectural to Decimal and the Insertion Scale to any metric unit you want to use.
OK and save.
2. You can then convert/edit the dimensions to metric.
Or create a new dimension style. In the Dimension Style Editor make set Primary Units - Unit Format to Decimal.
Set up all other necessary settings. Ok and set your new dimstyle as Current.
3. In your drawing draw the new dimstyle you created and match properties to all existing dimensions.
Note: Since there is slight discrepancy between imperial and metric system you can scale the whole drawing to make it exact according to metric system.