That's perfectly possible. Make a backup copy of the document in case you
do something you don't really intend to do. Then...
First, cleanse the text of all direct formatting:
Ctrl+A = Select everything (or select just what you want changed)
Ctrl+N = Apply Normal style format to the selection.
Ctrl+Q = Remove all non-style pargraph formatting.
Ctrl+spacebar = Remove all non-style character formatting.
Then apply the desired formatting using shortcut keys, style, or whatever.