Just went through this exact transition six months ago and wanted to share the specific pitfalls that cost us time and money. We bootstrapped for 3 years as an LLC before raising a $1.5M seed round.
The LLC-to-C-Corp conversion tax trap: When we converted from a multi-member LLC to a Delaware C-Corp, our accountant initially missed that the conversion triggered a taxable event for the existing members. The LLC had accumulated about $400K in retained earnings. Each member had to recognize their share of those earnings as income in the year of conversion, even though no cash was distributed. Make sure your tax advisor models this BEFORE you convert, not after.
The 83(b) election deadline is real: When we issued restricted stock to founders as part of the conversion, we had exactly 30 calendar days to file 83(b) elections with the IRS. There is no extension. Missing this deadline can result in founders being taxed on the value of their stock as it vests, which can be catastrophic if the company value increases between grant date and vesting dates. We almost missed it because our attorney assumed our accountant was handling it and vice versa.
Clean up advisor equity before the round: We had given verbal promises of 0.5 percent to two early advisors. During due diligence, our lead investor required us to either formalize those commitments with proper advisor agreements and vesting schedules, or get written waivers from the advisors. One advisor was cooperative. The other wanted 1.5 percent, which created a month-long negotiation that nearly derailed the round.
Bottom line: budget 3-4 months and $15K-$25K in legal and accounting fees for a clean conversion. Trying to rush it or cut corners on professional help will cost more in the long run.