A company currently has a SEP IRA and would like to convert it to a Simple IRA. Does one simply terminate the SEP and then adopt a Simple IRA or is the process more involved? Also, if they have deposited contributions to the SEP IRA for the partners for the current fiscal year, wouldn't they have to deposit any contributions to newly eligible participants before terminating the SEP?

Termination isn't necessary if it is just a SEP. Otherwise, amendment and notice to employees if SARSEP. Yes, employer must contribute to have SEP for this year. If SIMPLE adopted this year then SEP is no good and no contributions have to be made (but amounts contributed this year for owners in SEP are PROHIBITED EXCESS CONTRIBUTIONS. [All eligible employees must generally receive uniform contributions based on compensation.]