Use Edit/Find Change/Text.
First, create a character style, second apply it with find and replace.
Don't do that without a style.
Third, tell your coworker that he or she should not use T1 PostScript Fonts anymore, but OTF only.
@Kevin – as Ellis pointed out: This is a 2-step process. Or maybe a 3-step process.
Do all that on a duplicate of the original inDesign document in case something is going wrong.
Check before doing anything, if "Mercury Text G3" "Roman" or "Mercury Text G3" "Bold" is already used in the document with "Small Caps" disabled. If not, go on with the next steps.
1. I'd use the Find Font Panel to "Change All" instances of "Mercury G3 SC" to "Mercury Text G3" "Roman" and check the option "Redefine Style when Changing All" before pressing the "Change All" button.
2. Then I will go through all the defined paragraph styles and character styles that are using "Mercury Text G3" "Roman" and change the formatting to "Small Caps".
3. Then I'd use the Text Search/Replace for finding all instances "Mercury Text G3" "Roman" without the "Small Caps" and will replace the formatting with "Small Caps".
I think Laubender's approach is close to what I want to do. Unfortunately, Mercury Text G3 (Roman and Bold) are both used elsewhere in the document, so I can't Redefine the style when changing all.
Is there any way to add an additional (renamed) copy of Mercury G3 font to the font list which I then modify as noted?
@robokev I know its been a long time since this thread was posted but I've run into exactly the same issue and I'm wondering if you were ever able to come up with a solution?
Tell us more about your situation: What version of InDesign? What operating system? How is your situation similar to that of @robokev and how is it different?
@steve werner I'm using Indesign CC2015 on Win 7. SItuation is pretty much identical. I have old PostScript Small Cap font that need to be swapped out - in my case Scall San LC. Similar to what Robokev, mentioned in his last thread, Scalla San Reg, Bold and Ital etc are all used in this particular document so the steps Laubender recommended wouldn't really work out.