I see both options presented here as potentially viable. It is not unusual when a "key man" leaves a company for that person to continue to be involved for a transitional period. However, an up front statement that the entire book of business is in the air is also honest and ethical.
For a moment, look at your partners with a cold hard eye. When you are gone, will they continue to perform the work at the current level of quality when promised, or have you been consistently bailing them out?
If you have been bailing them out (outside of you doing your share of the work... they WILL have to hire someone or get another partner to maintain the same workload), then a transitional period will only disappoint the customers when you're no longer bailing them out. I would simply leave it with an up-front statement.
However, if it's more that they know you and because you handle to CS/QA, you're the face of the firm that is getting credit, a transitional period where a new face (to the customer) for the same work they have always received can allay their concerns and feeling they need to reanalyze their decision.
