Re:
Presuming that the 'old' and 'new' rims have a similar profile/depth it is possible to proceed as follows :
Undo all the spoke nipples until they are about to come away from the spokes
Place the 'new' rim alongside the 'old' rim ensuring the valve hole lines up and that any spoke hole/eyelet offsets are matching
Tape the two rims together, and working round the rim, undo and transfer each spoke and nipple across to the 'new' rim and
tighten all the nipples about 2 turns initially
Tighten the spoke nipples evenly until there is similar tension in all the spokes
At this point you will need to put the wheel in the frame to check that the rim is central and round etc etc ( It won't be ! )
You can now refer to Sheldon Browns' guide to wheel building or visit your LBS for help
Hope this gives you some ideas