You don't have to be ranked "Expert", you just need a guy who can blow things up. There're tons of expert in the game, just look for someone with a small bomb icon near his name. You have to have a free space to hire him though.
Yeah you would need to put a hit on the member u won't to get rid of to add a higher ranking member of what ever skill factor he may be, then you can find an advance member or an expert, etc.......