I do know that Accountants don't resign if they're getting paid and they don't feel in danger from association. It seems like common sense, but I have to admit I am drawing a blank right now as to citable references. Accountant resignation is not a terribly common occurrence.
They give up income when they resign and in much the way double-entry bookkeeping ingrains into them the need for an equal credit entry for every debit entry, they have to feel like they've got a really good reason for giving up the income.
There are some who would portray this as a "good" sign
I'd rank that up there with "Call the company". Can't imagine how an accountant resignation could ever be considered "good".
since changing to a small accounting firm after the resignation of one of the Big 5 would save money.
If a company fired one accountant to move to a cheaper one, I'd give that a "maybe". Doesn't wash if the accountant resigns.
Keep in mind the above is only my opinion and although it is based on years of observation and experience, it's still only my opinion. Wish I could cite some references. I would think the patterns that happen to companies after accounting firms resign would be quite well-documented somewhere.