It is, however, NOT a good automatic upgrade if you care about the credit utilization portion of your credit score.
The VISA Signature does not report a credit limit to the credit reporting agencies, even though BofA may tell you what your "limit" is, which can change frequently. It only reports your highest achieved balance. So if you pay off your credit card every month, or never keep much of a balance, it'll appear to the credit reporting agencies that you are using all of your available credit on the VISA Signature card (not a good thing for your credit score) -- even if you really are well below what you have available to use.
As a result of this, when I was automatically switched to the VISA Signature from my VISA Platinum a year ago, I talked with my personal banker at BofA and had them undo it. Instead, I was issued a new VISA Platinum Plus with a new number, but it preserved my reported credit limit to the agencies ... and truly indicated how much credit I had available that was unused.
I recommend the same approach to anyone who may want to maintain a higher credit score if they plan to get a mortgage, car loan, or the like in the next year or so.