easy way to replace via in board file

Tools->Padstack->Replace, then put in the old and new padstack names.  Can do for the entire board or just a single via.  You can also record these steps in a script and assign it a keyboard shortcut.

G'Day,
R²

I tried the first suggestion and it worked.
I have created a .scr per your suggestion, but when I run it the script replaces all the vias with the desired padstack.

On the script be sure to check "Macro record mode".

When recording the script: after inputing the padstack names and before clicking on "Replace", check the box "Single via replace mode".

when I try to do that the Single via replace mode is grayed out.

That's different from the version of 15.5 I'm running.  As soon as I enter the "Old" padstack name, that box isn't greyed out any more.  That's why I said to select that box after entering your padstack names.