Hello. I've already run underground about 50' of 3/4" conduit but have to pull out existing cable and replace with new cable. New cables will be: three #6 cables, one #8 and four #12's. I know it can be fished through but not sure if its code acceptable. I expect to upgrade my service panel some day which would bring things under the eye of inspection at that point. I hope I don't have to re-dig. Thanks

You will NOT be able to use 3/4 for that run. Check out the conduit fill table at Welcome to Western Extralite Company (http://www.westernextralite.com). The three 6's and the 8 already meet the available capacity.