I dont think the upgrade should cause problems generally but your software level jump is quite high.
I would suspect that your main problem is NAT configurations that have failed to migrate properly. We can already determine that traffic flows to other direction but not the other therefore the VPN connection is for the most part fine.
The easiest way to start troubleshooting this would probably to see the Main Site configuration and the configuration of one of the Remote Sites and try to get one of those connections working. After that the other sites would probably be easy to handle also.
There should not be any incompatibility issues between 7.2 and 8.3 for running VPN. So it should not be a problem running 8.3 at one site to 7.2 at another site. So the issue is probably something in the configuration of the ASA running 8.3. There were significant changes in 8.3 in the syntax for access lists and for NAT. I suggest that you do a careful review of the 8.3 config and look for something that did not transition correctly in the code upgrade.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...