Since I'm pretty far removed from the Match, I'm sure others can answer your questions more intelligently. However, to kick start the conversation, I'll throw in my two cents.
If any of the spots go unfilled in the SOAP would I be able to apply for them?
Since you did not participate in the Match, you are not eligible to participate in the SOAP. However, if a Rad Onc program goes unfilled after the SOAP, it is possible that they would solicit candidates from outside the Match. In this scenario, you would be eligible.
If I want to apply this cycle will I have to drop from my current radiology program?
Do NOT drop out of your current residency program until you have a signed contract for a Rad Onc position. I cannot emphasize this enough! Right now, you have a good future as a diagnostic radiologist which you should not compromise unless you are 100% certain you have something else lined up.
When and how do I have a discussion with my current radiology program?
This is a really tough situation and needs to be thought out carefully. I suggest that you try to squeeze in a "Rad Onc elective" to re-confirm if you really want to switch fields. If you still feel that Rad Onc is your calling, you have a range of options but none are really ideal:
1. Engage your Radiology PD early, tell him that you want to change fields if possible, request permission to interview. This is the most honest approach but your PD may use this information against you by not renewing your contract in favor of someone else BEFORE you have secured a position, that would be a disaster. Also, telling the PD that Radiology is not your preferred field may negatively color your future evaluations and rotations.
2. Stay mum until you are able to interview with a few Rad Onc programs. If you have a position, THEN tell your PD. This is the least honest approach but the safest from the perspective of your future career. On the one hand, if you are unsuccessful in obtaining a Rad Onc position then no one will be the wiser and you can carry on as if nothing happened. If you do get a position and leave then it may leave your fellow residents in a lurch in terms of coverage, rotations, etc.
What would you suggest be the best way about switching to radiation oncology?
In your situation, dropping out of your current residency and re-entering the Match doesn't sound like a good idea. I would look for residency programs that are adding spots outside the Match (you can look in this forum and the ARRO website, but content is nearly always cross-posted). Alternatively, try cold calling a few programs in your geographical area of interest.