An Effects Preset is simply an XML file that describes all the effects and parameter settings used for the "set" of effects you applied to a clip. It also supplies the Category for Final Cut to place it in.
You can right click on any Effects Preset and Show in Finder to access it.
If you have a text editor that will handle TEXT ONLY (I recommend BBEdit or similar "HTML" editor, but in a "pinch" I think you can use TextEdit — just be careful), you can open the Effects Preset file and reset the "category" to whatever you want.
This is the structure of an Effects Preset from the beginning of the document to line 14:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>$archiver</key>
<string>NSKeyedArchiver</string>
<key>$objects</key>
<array>
<string>$null</string>
<string>effect.video.filter</string>
<string>Effects Presets - Titles</string>
<dict>
<key>$class</key>
<dict>
(As far as I can tell, every EP starts out this way... with the exception of "Effects Presets - Titles" — that's a category I created for this EP.)
Where you see <string>Effects Presets - Titles</string> — on line 11 — that is the Category name for the EP. You can edit the <string> tag content there. If you don't see the category name at this beginning section, do a search for the category name. It will be in a simple <string></string> tag.
You CANNOT make changes to any tabs or whitespace in the structure of the file. You must respect the formatting.
After you make the Category change... on Line 11... you will need to restart Final Cut for the change to recognized.
I highly recommend that you BACK UP A COPY before you begin. If anything goes awry you can easily fix it by replacing the changed file with the backup. If (and when) you're confident about what you're doing, you probably can forego making the backups... I do.
HTH