Best way is to have them enlarged/reduced in a photocopy shop. You would need to know the change factor you require. To get this divide the scale you want into the scale you have. For example to convert from 1/12 to 1/32 you would need to change by 12/32 or 3/8 to go the other way the factor would be 32/12 or 2 and 2/3 (2.67).
Depending on the the factor it may be necessary to copy several times as the copiers usually have only a limited preset range of magnifications. This can introduce some errors in the resizing. It is best to mark a known length on the original against which the copies can be checked. It can also be a little expensive as you need to pay for each copy.
HTH
Doug
