Converting from an LLC to a C-Corp is relatively less complicated and less expensive than changing from an S-Corp to C-Corp. The emphasis is on ‘relative’ since both conversions involved legal and tax implications. You’ll, therefore, definitely want to engage a knowledgeable corporate attorney and a reliable accountant or tax lawyer.
On that tax front, LLCs and S-Corps are both pass-through entities with shareholders/members paying tax only once as owners. S-Corps, however, are more tricky and usually require more legal work (e.g., for converting and characterizing assets) than do LLCs that are shifting to C-Corp status. You can read more about how the IRS treats LLCs here.
In particular, the IRS scrutinizes how interests and assets are transferred between entities. The three chief methods are “Interests-Over-Conversion,” “Assets-Over-Conversion,” and “Assets-Up-Conversion.” You can read more about those characterizations here. While generally there are minimal adverse tax consequences associated with converting from an LLC to C-Corp, there can be some burdens such as those discussed here.
On the legal front, the complexity of the legal work and filings required largely depends on the state you’re currently incorporated in and the state where the new entity will be formed. If they’re the same state, that will certainly help to streamline legal work to some extent. State laws typically fall in one of three buckets regarding corporate structure conversions: (1) nonstatutory conversion; (2) statutory conversion; and (3) statutory merger.