In many cases client relationships are built and maintained by company representatives. These people, inevitably, progress, and have to be replaced.
It's advisable to notify clients well in advance of any changes regarding your representatives. They're the public face of your business.
Because of the amount of knowledge of the client's business they have, company reps are extremely important, operating as both advisors and as points of sale.
So some diplomacy is required when introducing a new face to your clients. They need to see their account is being treated as important.
Dear (use full title of the addressee)
I must inform you that our consultant / representative to your firm, (insert title and full name of person) will no longer be acting in that capacity as of (insert date of commencement of new rep).
In his / her place, (insert title and full name of new rep, include qualifications if appropriate) will be acting on our behalf as our consultant / representative.
(Name of new rep) has extensive experience in the industry, and his / her track record is impressive, so we consider him / her an appropriate choice of representative for our highly valued clients.