Transformer in reverse use?
Can i use a 30kva 3phase 480 delta to 208y/120 as 208 input and 420 output?


It may work subject to initial design of the transformer.
It's worth careful experiment - at first unloaded, and possibly with a small series load in the 208V feed to reduce maximum current - although providing a "small load" at those power levels may be challenging.  

You have specified 480:208 step down and 208:420 step up. If that is not a typo then you have allowed for the losses in the transformer by the lower required turns ratio which increases the prospect of success. 

Magnetising current per Volt should be about the same as before. 



Added: As Tony said and as above, some of the parameters MAY be different. Asking the manufacturer would be good if it is possible to do so.
Do you have a manufacturer's name and model number and can they be contacted? 

Failing that, my previous suggestion of "careful experiment" is likely to tell you enough. Applying initial "new-primary" voltage via a variac would be useful. The power rating needs only be a fraction of rated. If possible destruction of the transformer is acceptable then applying Vlv directly may be OK. It will probably be OK, but ... .