Charities are often concerned about changing their PR supplier for various reasons, including the following:
You can minimise the risk of the first concern through a rigorous selection process - asking potential consultants lots of questions, looking at examples of their work and speaking to references. Take the consultant on for a trial period - once they have worked on one or two stories you should have a good feel for the quality of their work.
To minimise any gap in PR support, arrange an overlap between the end of the contract with your current consultant and the new consultant/agency starting. To bring the new agency up to speed, invite them to visit your projects and set up meetings with all key staff. Collate information on your previous marketing activities, such as press releases, press clippings and newsletters, and provide these to the new agency.
The risk of losing media relationships is minimal. Although relationships help, journalists care most about the story - they'll bite off your hand for a great story, whether or not they've heard from you before. By using the internet, or by going through your press clippings, a consultant can easily find out what media outlets and journalists have written stories on your charity before, and will probably be able to suggest new contacts and publications that you haven't tried before.