Polystyrene properties after dissolved by acetone
What is happening is not actually the polystyrene melting but rather it dissolving in the acetone solvent. This is a physical change and not chemically altering the polymer. It should still retain its chemical properties as before.

However, other properties have changed. It is now a poorer insulator since the air pockets have been removed. It will also be more rigid for similar reasons.

The acetone ought to be able to be driven off by heating it in a ventilated space as acetone is quite volatile.