If you wish to reforge any elemental damage weapons from y1 they will be the default element, so if you have a gun you wish to keep a certain element, be sure to check your collections to see what element it will be reforged as before you dismantle it, if you care what element your y1 weapons are.

Yes. Unless you want to keep one of your masterworks to show off your amount of kills..... You can just rebuy each year 1 weapon at 20 light below your level....