Long story short, I confused 2 micros SD cards, and I format the wrong card, and then completely filled it with data. By the time I realized, it was obviously too late. My question, is the data that was on there completely gone and been overwritten with the new data? Any way to recover?

simple theory.. overwritten data can not be recovered...
Your case maybe can be recovered but of cource con not be 100%. Depend

it is 2 GB. if after format you use already 1 GB. then your chance is just 50%. if you use it until 2 GB. then No HOPE

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Was a 1GB Micro. I did use up the ENTIRE Capacity of the card (unfortunately). Any way to retrieve from any of the boot sectors?

No, there is no hope.
Maybe from your hard drive, where the original data is from.....

Janos

Absolutely no chance if you've filled it up again, 100% sure data is overwritten and gone.

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Might be a selling argument for a new type of memory:

OESC Memory: On_error_stretch_capacity

Just the question, how the memory should recognise that the
user is dum ... Perhaps one should implement a part of
Windows Vista software - every time one wants to write or
erase something on it, one is asked - "dear, dum user, are
you sure about what you intend to do?"

One might store huge amounts of data on a 1GB OESC SD card....
***

Sorry, but such an illogical question provokes an appropriate
answer ... How should it work - fill up a bottle with water, then
empty it, fill it up again with new water - and now there should
be still the old water somewhere in the bottle ????

+++

In this case data won't be recovered.
Good comparison with bottle, falter!