I would expect to see them as soon as the demand for S3's reduces. At the moment Bitmain are putting all their boards into the S3's but as soon as the demand drops they will have spare boards that will no doubt be offered as an upgrade kit for S1's.

I am just guessing here but would expect to have to replace the boards and move the heatsink across so you will probably need some new thermal paste, the S3 probably uses the same controller so just a firmware upgrade should fix that.

I have no idea how many batches of S3's there will be but maybe someone could help out by letting us know how many batches of S1's were made, this would give some rough guidance as to when the S3's will slow down and therefore when the S1 upgrade kit will be available.