Dear Miss Snark,
I'm preparing a query letter for my second novel. I have no pub credits to speak of, but my first two books (one of them non-fiction) were agented. At first I was going to mention this in my query (at the same making it clear that I don't want to market the earlier books again) but now I hesitate, for two reasons:
1--Agented though they were, the books didn't attract a publisher;
2--I don't think a query is the place to explain why I'm not still with the agents who represented me before, assuming that the agent queried wants to know.
Don't mention your earlier efforts. This is a new project, and you get to start afresh.
Besides, anytime someone tells me they ditched their first agent, it pings my Danger Will Robinson radar, and that is NOT a good thing in a query letter. Sometimes you have to mention it (like when you're shopping the SAME novel that was represented, or you have backlist) but if you don't, don't.