What happens if I reduce pm allocation?

[Nihonto]

Let's say I allow members 200 pm's. But now I want to introduce tiers, where regular members get maybe 100, and special members get 500.
If I change the allocation to members down to 100, what happens to those who already have more than 100 in their pm box? Do they keep them all and just can't do more, or will it delete the oldest ones until they only have 100?
I wouldn't want members to lose pm's that they already have.
Thanks in advance.

[DawPi]

They won't loose anything.

[Nihonto]

Thanks!