OK I'm a bit confused. Are you saying that in the master version it's just \p and you want to impose:
pmo - embedded paragraph opening
pm - embedded paragraph
pmc - embedded paragraph closing
Or is it that excessive use has been made of pm/pmo/pmc and you want to only alter how they are displayed in certain verses?
Styles are how you change the font, size, paragraph layout, etc. So, if the USFM is good semantic-based markup (e.g letters and other text where indentation is appropriate are marked up with pm*, nothing else is), then on the styling tab you can assign a font, size, etc. for that style. If you need to format Ezra differently to, say Revelation, then you can copy a style and make it apply to a single book, e.g. copy pm to id:GEN|pm (pm in Genesis - see the \id line for the relevant book).
If the problem is that the USFM is a total mess, then my preferred option 1 is talk to the translators/exegetical advisors, and get it changed in paratext.
Otherwise, (say if the original comes from DBL) you can use the changes.txt file to apply changes on a specific verse or range. E.g. to get rid of all "\pm"s from the rest of the file, and put them back in a few places, you could do:
at EZR "\\pm " > "\\p "
at EZR 2:2-4 "\\p " >"\\pm "
at EZR 97:124-132 "\\p " >"\\pm "
(note the spaces after the codes)
If you have multiple paragraphs that need handling separately, you probably need to do something like:
at EZR 2:2 "\\p Dear" >"\\pmo Dear"
You can also, of course, combine these 2 approaches, making a special "pletter" which sets the fonts and layout nicely, and then changing "pm" to "pletter" in in the relevant places.