I would like to know a better procedure for doing this, as the only way we have figured out is to escape and then go back into the same w.o. to straighten it out.  Here's what I have been doing:  Go to MFG-WO-K-A and enter the w.o. number.  Enter new Sequence Number, press ENTER, then do a lookup of Operation Templates.  
Click the one I want.  Now here is the first place I get stuck - F10 doesn't work to get out of the listing, so I Escape.  
Now I only see 0 in the Operation Field instead of the one I had selected, and pressing Enter doesn't work.  The only thing that works is to press Escape again and go back into the line - but the Seq number I manually entered has been changed to match the previous sequence number, so I have to overwrite it.  
If anyone has a better way I am eager to know it!