Woah. Lots going on here.
Yes, you can use a transformer in reverse, but you need to be really careful.
1) Only some transformers are permitted by the manufacturer to be reversed. They will probably work if not specifically permitted, but....
2) reversed transformers have much worse inrush characteristics. Depending upon what is feeding the transformer this might be a problem when you energize the transformer.
3) When you use the delta as second don't have a neutral available. This means you cannot ground in a balanced fashion. For loads such as motors or resistance heaters this doesn't matter. But VFDs really should be fed with a grounded wye (grounded neutral) source.
4) You can corner ground a delta source. It still works just fine because the L-L voltages are unchanged. The 'grounded b' has 0V to ground, but 480V to the other phases.
Jon