You cannot just transfer the S-Corp return (1120-S) into a C-Corp return (1120). You need to file a final S-Corp return and start an initial C-Corp return.
SuperUser Critter#2 offers the following advice:
I highly suggest you seek local professional assistance in filing the final & first returns so you get them correct. This is usually a tricky situation so professional help is well worth the money since getting it wrong can cost much more in penalties & interest later.