All IM positions through the Match start at PGY-1, so you would need to repeat your entire intern year if you get a categorical position through the Match. You won't get any credit for any time since IM programs will be counting on your to staff their services for the entire year. Also, you probably won't have enough time to interview for both IM and EM positions during a busy intern year.
If would reapply to just EM through the Match this time. In case your don't match again and are unable to find a spot by around March or April next year, there are plenty of PGY-2 IM spots that open up usually around March-June every year outside the Match since this is time that Categorical PGY-1s decide not to renew their PGY-2 contract (eg they are transferring to another IM program or are switching specialties altogether). You may be able to stay at your current program for PGY-2 year if a spot opens, but if not you would need to transfer to another IM program with an open PGY-2 spot. The main difference between the Categorical and Prelim PGY-1 IM schedules at most programs is that the Prelims don't have any significant outpatient (ambulatory clinic) blocks, so that would be new for you. Remember you if you like EM, you can always do an EM type fellowship after IM residency to get more EM training and then work shifts in an ED (as an internist with ED experience) so it doesn't have to require doing a dedicated 3 or 4 year EM residency.
The bigger question is why you don't think you didn't Match EM this year. Making the necessary improvements in your application this time will be crucial; otherwise expect to yield the same results if applying with the same application. Are you USMD, DO, or IMG? How many programs did you apply to, how many interviews did you go to, and how many programs did you rank? What are your Step scores? Any red flags? Did you meet all the EM LOR requirements (eg SLOEs? EM has actually gotten slightly easier to get into lately (with USMD Match rate of 91.6% and mean Step 1 score of 233) since there are a lot of new community programs that aren't very competitive to get into.