OK, Manks,

Here's what can or will happen when you put a .61 into a .40 trainer.

Strength:
1. Rubber banded on wing. Not very strong. If you yank hard, you can lift the leading edge of the wing. It will scare the hack out of you. In particular, install the rubber bands from FRONT TO BACK. This stretches them tighter in the front so they hold the leading edge down better.
2. You wing has balsa spars, more than likely. Making a turn or pitch maneuver at a higher speed puts more g's on the plane. There is a possibility that you can break the wing.
3. It has a thin wing. Sticks tend to have a fairly thick, symmetrical airfoil, making then inherently stronger than a thin wing.
4. Center joint. Most trainers do not have a big wing joiner. You need to use fiberglass cloth 4" wide or so and 30-minute epoxy on the top and bottom of the joint for added strength.
5. Stab & fin. Can be easy to break off on a trainer with high g maneuvers. I recommend guy wires. You can use weed eater line. It's cheap, easy and plenty strong enough.
Servos and pushrods. You standard seervos will be OK. Check the pushrods to make sure they can't flex and allow the controls the straighten out.
6. As has been mentioned, reinforce the firewall using 30-minute epoxy. the 30-minute kind soaks in more and gives a stronger bond. Use tri-stock or 1/4" square in the corners.

Aerodynamics:
1. Flat bottom airfoil. As you increase the speed, your plane will tend to climb. When you reduce power to land, you'll need to hold a good bit of up elevator or retrim. I recommend raising the trailing edge of the wing with 2 popsicle sticks to reduce the incidence and alleviate the clim.
2. Your CG may be out. Rather than add weight, move the elevator and rudder servos to the read fuselage. Cut a hole, add a little piece of ply at each end and screw them in place. Doesn't have to be fancy, just has to hold.
3. Wing tips. If your trainer has plastic or wood wing tips that are beveled upwards, take them off. They act like more dihedral, which you don't need.
4. Flutter. Keep an ear open for a buzz. Most of the time it is the aileron tips. Try cutting the tips off at an angle starting 3" in from the tip to a point at the tip.

Engine:
1. Use a low pitch prop. No more than a 6, but a 4 pitch is better. It will give you the climb, but not the speed which will stress the plane. A low pitch prop will also help you slow down for landing.
2. Use power management like the guys with the really big planes. Half power in level flight and full power for straight up climbs.

Other:
1. You may need a longer landing gear or larger wheels for prop clearance.
2. Since you are changing to a tail dragger, don't glue in the new gear block with CA. Use epoxy and tri-stock in the corners.