Which couplers should I use? What are the steps in changing from Rapido to Knuckle
Use Micro-Trains #1128 for this loco. Simply pry off and discard the rapidos along with the coupler boxes (the MT's come with new coupler boxes to fit in place of the original ones, they simply snap back into place). Simple follow the instructions included with the MT couplers and in less than an hour you will be good to go...
Unless something has changed recently, this is not a straightforward switch. You may need the MTL 1134 coupler kit (for Bachmann Plus F's) or the 1133 kit (for Bachmann Spectrum F's). The kits contain replacement coupler boxes along with 1128 couplers that go in them. The MTL website conversion chart says to use 1128's for "new" Spectrum F's, but the newer packages of 1128's I myself have do not contain the replacement coupler boxes. I can see why you asked!
Here's the 1133 diagram:
(http://www.nscalesupply.com/images/mtl/MTL-1133Diagram.gif)
Here's the 1134:
(http://www.nscalesupply.com/images/mtl/MTL-1134Diagram.gif)