Serial# Shows Date, Regrade?

the serial number doesn't tell you any of the information you asked about, all it allows you to do is check to make sure it is a legit certified coin and not a counterfeit slab. There is also no way to tell when it was graded by serial number but there are some differences in each generation of slab (they change every now and then to try to stop counterfeiters) that will tell you in a time range it was graded but thats all

The grading service can tell from the serial number specificly when it was graded. Their records miht tell them about regradeds crossovers etc but I can't guarantee that. Unfortunately they will not give out that information for the very reason you guessed.

If it is a PCGS or NGC slab I han help you out with links to threads that will tell you what generation they are and the approximite date range when they were slabbed. For other companies you will have to talk to me directly.

PCGS slabs
PCGS slab varieties

NGC slabs
NGC slab varieties