Years ago, I played under a DM that had several things that he said he had copied out of others books for D&D 3.5. And one of those was a magic weapon special ability that allowed you to change the weapon material for the purposes of overcoming damage reduction. Is that an actual ability and if it is is there a Pathfinder version of it?
There are two 3.5e weapon enhancements that fit the description, both priced as a +2 equivalent. Metalline is probably the one you're thinking of, and allows the wielder to change the weapon's material to silver, cold iron, or adamantine as a standard action.
Transmuting weapons are similar, and generally a stronger option. This ability causes the weapon to alter itself upon hitting an enemy (it's an automatic process, and can't be manually activated by the wielder). On subsequent attacks against the same target, the transmuted weapon will ignore all forms of damage reduction, not just material-based DR.
I'm not aware of any Pathfinder equivalent for either ability, so they're only available if your GM is willing to allow 3.5 content.
Transformative weapons can change damage type.
Among the magic weapon special abilities in Pathfinder, none of them let you change a weapon's material at command. Some special abilities (e.g. grayflame) allow the weapon to overcome certain material DR, but they do not actually change the weapon's material.
As for damage type, a transformative melee weapon can be commanded to reshape into a similarly sized melee weapon, even if the new form has a different damage type. The weapon retains this new shape until unattended.
This special ability can only be placed on melee weapons. A transformative weapon alters its shape at its wielder’s command, becoming any other melee weapon of the same general shape and handedness; the weapon’s categorization as simple, martial, or exotic is irrelevant. For example, a Medium transformative longsword can take the shape of any other Medium one-handed melee weapon, such as a scimitar, flail, or trident [...]
The weapon retains all of its abilities, including enhancement bonuses and weapon special abilities, except those prohibited by its current shape. For example, a keen transformative weapon functions normally in the form of a piercing or slashing weapon, but cannot use the keen special ability when in the shape of a bludgeoning weapon. When unattended, the weapon reverts to its true shape.
The greater transformative ability is similar, but with fewer restrictions.
A greater transformative weapon becomes any other weapon the wielder desires when a command word is spoken. The weapon cannot become ammunition, but can freely change between simple, martial, exotic, light, one-handed, two-handed, melee, and ranged. The weapon retains all of its enhancement bonuses and weapon special abilities.
However, abilities prohibited by its current shape do not function. [...] When unattended for 1 day, the weapon reverts to its true shape.