So here's goes.

Let take LS Pete Alonso for example.

When you view his card you see that he has 51 Fielding.

Now go over to where you can Mod him and Select Fielding Diamond.

After you select that option there is an Attribute Boosts display in the bottom right.

On Defense it shows that Fielding is mow 58.

That means his OVR Fielding is now 58.

But does that mean the 51 Fielding that was previously shown when you first view his card now changes to 58?

Or does it stay at 51 and you just have an "unseen" boost?

I also assume that once hes P5: that 51/58 Fielding OVR may also increase.

It just shows 51/58 now because he hasnt achieved and parallel/mod boost.