I refinished a pre ban lower, the SN was already lightly stamped and now it barely shows up same with all the stamping on my lower. what legally can I do? Leave it alone, or re--stamp it?

Easiest way to bring the number out more clearly would be to fill it with white enamel paint or the like - fill and wipe and let it dry.  The SN will stand out like a beacon.

As long as you're not removing or altering the original markings, it should be OK to touch them up for better definition.

I had to call ATF on a similar question from a customer who had a slide refinished, and the markings were very faint afterwards.
ATF said that if the required markings were  not legible, they would have to be restamped. Guess that boils down, for the moment, to your definition of "legible". The ATF is working on requirements for  minimum stamping depth for weapons serial numbers due to the number of manufacturers and importers who are bringing in some weapons with almost illegible numbers and markings.

These are almost non existant, so I attempted to re-stamp. Now looks like shit, but you can definetly see the numbers. I don't know what to do now?

If there is any question in regards to tampering with a serial number, the NFA Branch has in the past had the ATF Forensics Lab (and sometime with the assistance of the FBI) x-ray the receiver for evidence of tampering. They have also done this to identify improperly demilled gov't M16s(read: stolen)that where later registered during the NFA amnesty as being "rewelded".
That said, I would think the ATF could do the same if your weapon was "under scrutiny" so-to-speak. THey have all kinds of tricks up their sleeves.
Afterall, it's not like you tried to file the number down in order to avoid being traced/tracked.
New firearms are subject to ATFs policys on proper depth and size (I think this is currently being modified.)
Hope this helps?
-rigid

Stamp away! I wouldn't worry about restamping- a simple acid bath test will tell the investigator whether the stamped numbers have been altered or not from the originals.
Semper Fi