1. If I was an SSI DiveCon and I wanted to continue with PADI on my way to being an Open Water Instructor would I have to go into the PADI AI course or right into the Open Water Instructor class?
As has been pointed out, AI is part of the IDC course and in this case you would have to do a full IDC.
In addition, there are a couple of extra sessions - for example "teaching the PADI RDP" becomes mandatory rather than optional, so you will find that you need a bit of extra time. It would also be worth familiarising yourself with the way that PADI teach/assess dive physics, physiology etc. Also, have a look at the skills that PADI teach and make sure you can do this skills - I'm not familiar with the SSI curriculum so can't point out any differences myself, sorry.
2. If I was an SSI Open Water Instructor and wanted to switch to PADI what would I have to do? How much and how long?
The minimum amount of time is four days to complete the OWSI portion of the IDC, though with the same provisos as above - there will be some extra familiarisation work needed. The IE is another two days.
How much? Depends on way too much to really answer. You will need to buy a full set of PADI instructor materials (rough guess - US$500), pay for the IE (another $500 or so) and your PADI membership ($300?) as well as the tuition. Here, the tuition would work out as around US$750 or so.... depending on the CD. Take all costs with a pinch of salt - they vary around the globe and I've guessed from the PADI-AP costs what is likely.
My honest advice would be to decide which agency you want to teach for in advance, and enter in to that training in good faith. Worry about a crossover if you ever need to, but planning for it is nuts..... if it's a real concern, then go in that direction now as it will save you time and money.