Your SEP IRA custodian can tell you if they'll allow you to submit an updated 5305-SEP for your existing SEP IRA with the LLC's information or if they want you to open a new one.
Assuming you have no employees, your S-corp can contribute up to 25% of your W-2 compensation, which is deductible on the S-corp's 1120S and not your personal return (it all flows through to you anyway).
Only you as the employer can make contributions to your SEP IRA. Employee deferrals aren't allowed. This means that if you wanted to contribute up to the $66,000 limit for 2023, you'd need W-2 compensation of $264,000.
Contrast that with a Solo 401k (only if you have no non-spouse employees), which allows both employee deferrals (up to 100% of compensation up to $22,500) and employer contributions (up to 25% of compensation to a combined maximum of $66,000). This lowers the compensation needed to max out your contributions, meaning a larger tax deduction is possible with lower income.
There are definitely pros and cons of using a SEP IRA vs. a Solo 401k to consider. Here's a few:
SEP IRAs have fewer administrative requirements. For example, I had
to do a plan restatement in 2022 for my Solo 401k in order to
maintain qualified status with the IRS. Additionally, Solo 401ks
with more than $250,000 in plan assets need to file form 5500-EZ
annually. Neither is too bad, but that's two more items compared to
a SEP IRA.
SEP IRAs are counted in pre-tax IRA balances for the purpose of IRA
aggregation/the pro-rata rule. 401ks aren't counted, which is
generally seen as a good thing for those considering
non-taxable/backdoor Roth IRA conversions.
Solo 401ks can allow Roth
contributions (custodian-dependent), which SEP IRAs couldn't have until Secure Act 2.0 introduced
Roth SEP IRA accounts starting in 2023. That puts them more on par with
401ks in that regard. However, this is a recent law and some provisions have not yet come into effect and the IRS hasn't published any guidance or regulations on the matter.
A SEP IRA plan can be continued even if you hire employees, but a
Solo 401k must be shut down or converted to a Traditional 401k plan
if you have non-spouse employees.