A workaround you can use is to delete the style file the level currently uses (or - move it out of the "styles" folder, then move it back afterwards). When you try to open the level, it'll ask you to select a style to use in place of the one that it now can't find.
The more complicated variant, for if you just want to change *some* of the pieces, would be to cut and paste them to a new level, do that, then cut and paste them back to the original one.
(Depending on how the new formats work when they're implemented I suspect it may be possible to do what I want to do with with a Find/Replace operation in Notepad but I'd have to see)
When the new formats come into use, it'll be possible to do this. Currently, the old formats are still being used for everything apart from replays.
Here's a couple of samples of how things look in the new (text-based) format:
$OBJECT
COLLECTION default
PIECE window
X 96
Y 40
NO_OVERWRITE
DIRECTION right
$END
$TERRAIN
COLLECTION dirt
PIECE moss_06
X 865
Y 114
NO_OVERWRITE
ONE_WAY
$END