Converting from traditional aluminum siding to smooth skins?

If it was me, I would remove the roof, rebuild the walls using foam block insulation, rebuild the roof, then reinstall the existing metal. It goes on real easy and is very durable. Especially with it looking so good.

Check the floor while you are at it as it may pay to do a complete rebuild using what you have as the pattern.

One thing to try and avoid is aluminum contact with the wood. And make sure you don't use treated wood because though it seems like a good idea, the chemicals love to corrode the aluminum.