Some good advice in the initial answer here...
http://uk.answers.yah...20100326100417AAaG5U7
Make sure you have ok'd everything with a new solicitor before disinstructing the old one so there is minimum disruption to your case and bear in mind practicalities eg if there is an imminent hearing, any new solicitor needs adequate time to get any relevant paperwork and prepare properly.
Get their advice as to any changeover. They may well sort the appropriate forms.
Note the points about them being able to take on Legal Aid work as not all firms can. Also bear in mind rights of audience (which courts/at what level an individual solicitor can represent you in).