When you replace a bathroom sink, the replacement sink should be the same size or slightly larger than the original sink for a secure fit. Double-check the measurements of your old sink before purchasing the new one. Measure the length, width and depth of the old sink. Then Measure the width of the countertop. Bring the sink and countertop measurements with you to the store. List the hardware you need or plan to replace: faucet, trap, tailpiece and supply lines. Turn off the water supply. In most bathrooms, the shutoff valves are located under the sink. If not, turn off the water at the main water-supply valve. Test the faucets to make sure the water is off. Place a bucket under the trap. Loosen the slip nuts on the trap and remove it. Empty the water from the trap into the bucket. If there is a tailpiece connected to the drain assembly, remove it as well. Disconnect the supply lines from the faucet. All of the plumbing should now be disconnected from the sink. Removing A Counter Top Sink If the sink has clips, loosen them from underneath the counter. If the clips won’t detach, break them off and throw them away. Be careful not to damage the counter. There should be a bead of sealant between the counter and the sink rim. Cut the seal along the rim with a utility knife or wedge it apart with a small pry bar. If you use a pry bar, put cardboard or a rag under it to protect the counter. Lift the sink straight up out of the counter. You may need a helper if the sink is too heavy. Nowdays, people usualy makewainscoting in their bathrooms. If the sink and countertop are a one-piece unit, there should be sealant holding the countertop to the vanity. Use a utility knife to cut the seal. Be careful not to chip the vanity when breaking the seal. Remove the countertop and sink. You may need a helper if the unit is heavy. Removing a Wall-Mounted Sink The sink may be heavier than you realize, so ask a helper to hold the sink while you disconnect the plumbing. If the wood behind the sink is rotten, the plumbing may be the only thing holding it up and disconnecting the plumbing could cause it to fall. If the sink is mounted in a bracket, you should be able to lift it out as soon as the plumbing is disconnected. If the sink is bolted to the wall, have a helper hold the sink while you remove the bolts. Once the bolts are removed, the sink should be free. Installing the New Sink Install the faucet in the new sink. Put a ring of plumber’s putty on the drain flange inside the sink. Insert the drain assembly and install any washers provided. Tighten the locking nut until putty squeezes out from the drain flange. Wipe away excess putty. Turn the sink upside down on a towel or other nonabrasive surface. Run a 1/4" bead of silicone or latex caulk under the sink rim. Carefully lower the sink into position. Press down firmly until caulk squeezes out around the rim of the sink. Wipe off excess caulk. If the new sink requires clips, install them at this time. To install a wall-mounted sink, see Installing a Pedestal Sink.

I've got the same problem. My supply lines are welded with no nuts or bolts.