Depends if they're loctited or staked in place. Loctite usually yields to tools, but heat can help. Don't ask me how much to apply, I've always had enough luck with tools. Staked bushings often need the staked areas ground a bit to free them. If you're not comfortable doing any of this, see a smith and avoid potential damage to your frame's threads.
If you have Brownells tool for removing them, you can loosen up red Loctite by putting a soldering gun tip into the bushing and turn it on for a minute or so. Usually breaks free.
If it is a case of being staked (look at the back side for staking marks) you have to do it by main force and awkwardness. Putting a cheater rod on the tool helps.
What are you trying to take out the bushings for?? If the new thin grips you ordered require shorter bushings just file the tops with a smooth flat file until the screws lock on the wood and not on the bushing. Remember not on the bushing