If the company terminates the 403(b) plan, which is likely a requisite to establishing the SEP plan, your assets in the 403(b) plan will have to be distributed from the plan in a lump sum. Since, by being still employed with the company, you will not have reached your required beginning date for distributions from the plan, I believe that the entire distribution will be eligible for rollover to a qualified plan at another employer (and you'll likely be able to continue to avoid taking RMDs while still employed there), or to an IRA (perhaps to the SEP IRA).
Any IRAs that you have, including the new SEP IRA, will be subject to RMDs.
You'll want to confirm this with your 403(b) plan administrator.
