Ideally you'd find an advanced spot in the SOAP. The next option is you reapply during internship for "R" positions (positions that start for the coming July, typically available due to a resident deciding not to pursue the advanced position they matched in anymore) and for normal advanced positions (will start the following July). You could apply for categorical positions as well, but that would mean doing an intern year again, though that does give you a job during the "glide year" in the event you can't find an R position.
Most prelim programs will be understanding and try to accommodate you if you need to go on interviews, assuming you let them know in advance so they can try to design the rotation schedule to put you on a rotation where you can take vacation without putting too much stress on the program/other residents. Ultimately it's on a case-by-base basis.
The key is to not get in this situation in the first place of course. Applying as an intern puts you at a disadvantage compared to applying as an MS4--you want to match the first time around. That means if you don't think you're a competitive applicant, to apply very broadly. It's better to apply to too many programs than too few, as you can always cancel interviews.
