If you merely swap valves, the old tank can be recertified for (I think) five years and that's it (might be less). New tanks are available at Home Depot for about $22 and are good for 12 years.
Inspect the new tank selection and you will find the individually marked tare (empty) weights vary in the neighborhood of 14-15 lbs. If you want to keep your weight
down, select a lighter set of tanks.
If you really want to keep your weight
down, just get one tank and also a 'steak saver' or similiar propane
fitting to allow attachment of a one-lb cannister for when the big tank runs out.
If you want to save even more weight
(I did this, but don't have a reefer or run my furnace
for long, so I don't use much propane), put in one ten-lb tank, plus the steak-saver.
Pete in the RatHaus