You can use regular expressions, so if you can build the expression, you should be able to use it.
But instead of doing that, do you REALLY need the element to be an h2 for semantic reasons, or are you just trying to increase the font size using browser default behavior? If it’s just for the font size, why don’t you do a mass search/replace of the whole text and change it to a class that you can style to your hearts content? It’s going to be a lot less burdensome in the long run…
If you’re using headings for font sizing, you’re using them wrong. They are used to show the importance of the heading, H1 being the most important, H2 slightly less important and so on.
How they display is completely a manner of styling. You can make H6 look bigger than H1 - doesn’t necessarily make sense, but you can. But semantically, there should be a purpose to which heading element is chosen.
I specifically asked the OP if he was changing to an H2 simply to use the base browser behavior to get a bigger font size. If that’s the case, then I was arguing there was an easier way.
However, if there is a semantic reason to have the H3’s changed to H2, then by all means, he should do so.
But considering the H3s are styled as display:inline, I’d guess that it’s more a text size issue, which is NOT a significant right reason to change an H3 to an H2.