We reincorporated our business late last year from an LLC (Colo) to C Corp (Dela). Obviously tax treatment is very different. Do we file as a C Corp, file twice, etc?

LLC is not a business type for taxes, if you were taxed as an LLC partnership or sole proprietor and then you created the c-corporation.  Yes you file for part of year as the partnership or sole proprietor (final tax filing)

Then you start a new company file as a corporation.

If you were taxed as an s-corp to start with, and converted to a c-corporation, you really need the help of a tax professional.  You do have to file a revocation of the s-corp status with the IRS, and there are some other requirements too.  get professional help if this the case