Elemental spell clarification

So my sorcerer took Elemental spell (acid), and I'm a wondering if I'm reading this correctly. It appears that
1) Any spell that does damage can have this metamagic feat applied to it
2) The spell descriptor doesn't change even if it had an elemental one already. So [fire] doing cold damage is still [fire].

Is this really what the designers intended? Some people that I've talked to seem to think that the metamagic feat can *only* be applied to spells that have an elemental descriptor - acid, fire, cold, etc. And does it really make sense that after applying the feat, it doesn't change the descriptor.

The secondary question is if say you change stone call to acid stone call, the secondary effect of the spell is difficult terrain made so by rubble. But in this instance, is the area covered in acid. Should it do any damage? Should it still be difficult terrain?

Benefit: Choose one energy type: acid, cold, electricity, or fire. You may replace a spell's normal damage with that energy type or split the spell's damage, so that half is of that energy type and half is of its normal type. An elemental spell uses up a spell slot one level higher than the spell's actual level.

The Elemental spell Feat changes 100% or 50% of a spells Energy damage.

Spells that do Percing, Bludgeoning, or Slashing damage are not affected.

A rain of dirt, gravel, and small pebbles fills the area, dealing 2d6 points of bludgeoning damage

I can't agree with Dr. Styx. I cannot see any requirement that the base spells damage be elemental in nature. You can (for example) replace bludgeoning damage with fire damage with this feat. As you can see if I bold slightly different sections of the feat.

Benefit: Choose one energy type: acid, cold, electricity, or fire. You may replace a spell's normal damage with that energy type or split the spell's damage, so that half is of that energy type and half is of its normal type. An elemental spell uses up a spell slot one level higher than the spell's actual level.

A spell should gain the [type] of whatever damage you add to it, losing the old type if you replace 100% of the damage.

With regard to acid stone call, it won't deal additional damage, but will leave behind difficult terrain because the feat doesn't alter secondary effects. Maybe the acid isn't concentrated enough to burn, but does make the ground slippery.

I agree dragonhuntereq, it seems to me that it just allows you to replace the normal damage of a spell, no where does it make reference to it requires the spell to be an energy spell.

By RAW, it looks like it neither gains the appropriate descriptor nor does it lose any of it's previous descriptors, so a cold fireball, only has the [fire] descriptor, which seems patently wrong.