Applescript changing value on plist
Firstly, you can simplify your initial script—the one used to create the property list file—to this:

    set plistR to {theName:"name", creationDate:"date", serialNumber:"serial #", fullEmail:"@", IPAddress:"1.2.3.4"}

    tell application "System Events"
        set plistf to make new property list file ¬
            with properties {name:"~/Desktop/MY_DATA.plist"}

        set plistf's value to plistR
    end tell


Then changing a value is as simple as changing an item in the record plistR, and setting plistf's value to the new plistR:

    set plistR2 to {theName:"name2", creationDate:"date", serialNumber:"serial #", fullEmail:"@", IPAddress:"1.2.3.4"}

    tell application "System Events"
        set plistf to the property list file "~/Desktop/MY_DATA.plist"
        set plistf's value to plistR2
    end tell


If you don't want the hassle of writing out a whole new record declaration (imagine you had, say, 1000 items and only needed to change one), you can adjust a single plist item also like this:

    tell application "System Events"
        set plistf to property list file "~/Desktop/MY_DATA.plist"
        set the value of the property list item "theName" of plistf to "name3"
    end tell