Actually, it didn't start life as an upgrade incentive.
First there were people who had bought a 3 year contract on some flavor of S3s at a time when PLS (Product Lifetime Service) wasn't being offered, and their 3 years was ending and they were calling to not renew service--They got offered the deal because that was $99 more than TiVo was going to get otherwise.
Then it turned into people buying S4s and wanting to cancel the subs on their S3s, and again TiVo decided another $99 was better than $0.
Some people buying S4s even managed to play CSR well enough to get the deal on S2s they already had.
When word got out it probably resulted in some people buying an S4 sooner than they might have otherwise in order to lifetime their S3s, so it turned into an upgrade incentive, and there were some offers later for particular S3 owners to get them to spend more money.
But apparently you need to have owned and had under subscription the S3 or S2 for several years in order to get them to give you the $99 deal when buying an S4, except perhaps for a few people who got lucky on CSR.
Anyone looking to get a break on lifetime for an older unit should talk to TiVo, Inc. first, before spending any money or agreeing to an X months subscription commitment on a new TiVo, to get the details nailed down and get a firm commitment from them.
"I am altering the deal. Pray I don't alter it any further."
Darth TiVo, 14 February, 2011